The beginning contradicts the end; we rule like Rabbi Yochanan that nighttime is a time for tefillin.

Let me explain in more detail.
In Menachot 36b, the tannaim disagree about the source of this law: Rabbi Yosei HaGelili learns that nighttime is not a time for tefillin from the same verse from which Sabbaths and holidays are excluded as times for tefillin: “And you shall keep this statute in its appointed season from days to days.” Rabbi Akiva derives the exclusion of Sabbaths from a different verse: “And it shall be for a sign upon your hand.”
Maimonides apparently rules like Rabbi Yosei HaGelili, even though regarding the exemption on Sabbath and holidays he brings Rabbi Akiva’s source. It is Maimonides’ way to cite a source that seems correct to him even if it is not the one accepted in practice. You can see this from the fact that he rules that there is a prohibition against putting them on at night. And the Talmud there explains that this is a negative implication attached to a positive commandment, which for Maimonides apparently counts as a prohibition.
But in the Shulchan Arukh it seems he rules like Rabbi Akiva, and therefore wrote that the nighttime prohibition is rabbinic, lest one sleep. (The positive commandment and its negative implication in that verse do not deal with tefillin but with Passover.)
It should be noted that on 36a there, the tannaim also disagree about until when one may wear them, and in practice it seems the ruling is until sunset, against the Sages who ruled until bedtime. On the face of it, that does not fit well with the Shulchan Arukh, unless perhaps we say they made no distinctions.
In any case, it appears that the Shulchan Arukh ruled like Rabbi Akiva. Therefore one should not put them on at night only because of a rabbinic prohibition. Still, it does not necessarily follow that according to his view, one who puts them on at night has fulfilled a commandment. It may be that even according to him there is no commandment at night, but one who puts them on has not violated a prohibition. And perhaps that is also the explanation of Maimonides, but this is not the place to elaborate.
At any rate, it is clear that according to all views one does not recite a blessing over tefillin at night, even if nighttime is fundamentally a valid time for tefillin. That is because the Sages prohibited putting them on at night. This is true not only according to Tosafot on Sukkah 4a, who hold that someone who did not comply with the rabbinic requirement has not fulfilled the Torah-level obligation, but even according to the Ran, who disagrees with them; it is still clear that rabbinically one should not put them on, and therefore no blessing was instituted for doing so.
That is why I added that in any case, in practice nighttime is not a time for tefillin and there is no point in putting them on then (not necessarily at the Torah level), and the blessing is in vain.

It should be pointed out that in סעיף 3 the Shulchan Arukh writes that someone who gets up early to set out on a journey may put on tefillin even at night, since there is no concern that he will sleep in them, so the rule that one does not put them on at night is not absolute. And Igrot Moshe, Orach Chaim 1:10, wrote that he may even recite a blessing over them. So it’s hard to say this is a blessing in vain.

That’s unrelated. When there is no concern that he will sleep, perhaps they did not make the decree, and then one may put them on and also recite a blessing. That would not fall under the decree.

But Igrot Moshe’s reasoning is also that he links Rabbeinu Peretz’s view (in סעיף 3) that one may recite a blessing before dawn with the Rosh’s view that one may put them on at night from the outset, except that this is not taught publicly as a practical ruling (even at the beginning of the night). Even according to Igrot Moshe it is preferable not to recite the blessing; I’m only saying that it’s hard to call it a blessing in vain. True, we do not generally rule that way, but Igrot Moshe relies on it in pressing circumstances, and several medieval authorities (Rishonim) held this way.

Obviously, if nighttime is not a time for tefillin, then one should not put them on and it is also forbidden to recite a blessing, even if there is no concern that he will sleep. What does that have to do with our issue? Obviously he is following the view that nighttime is indeed a time for tefillin by Torah law—that is exactly what I wrote. And still, it is obvious that in our case this is a blessing in vain. I don’t understand what is new to you here.

According to the Rosh’s view, and the medieval authorities who had the reading that Rav Ashi put on tefillin from the outset after sunset, except that this is not taught publicly as a practical ruling, it is obvious that this is not a blessing in vain. That means not only that nighttime is a Torah-valid time for tefillin, but also that the rabbinic decree is not absolute. The Rosh really does seem to contradict himself, but in any case Rabbeinu Peretz holds this way according to Igrot Moshe. I understand that the Shulchan Arukh did not rule this way, but Igrot Moshe relies on it in pressing circumstances, so it is not clear to me why it is obvious to the Rabbi that “in our case” this is a blessing in vain. Even at twilight many halakhic decisors say to recite a blessing for this reason. Does the Rabbi disagree with Igrot Moshe? Does the Rabbi hold that this approach among the medieval authorities is completely rejected? It does not seem so.

I explained. If they decreed not to put them on, then even if it is only rabbinic, there is no logic at all in reciting a blessing over that. One can discuss situations where the decree does not apply, such as in the morning when there is no concern that he will sleep in them. Even there it is not so reasonable, because in the end they did make a decree. But in an ordinary nighttime situation there is nothing to bless over. Simple and obvious. If the ruling is that there is no decree, then there is no decree. (I don’t know what an “unqualified decree” is supposed to mean.) But in practice, there is one. I don’t understand the hair-splitting here.

I didn’t say one may put them on at night, nor that one may recite a blessing at night—only that if he already recited the blessing, it is not a blessing in vain. If in pressing circumstances Igrot Moshe relied on the opinions that there is no sweeping decree, then certainly after the fact you cannot call it a blessing in vain. You can disagree with those medieval authorities and with Igrot Moshe, who relied on them; I’m only saying that they exist.
Responsa Igrot Moshe, Orach Chaim, part 1, siman 10:
“For according to the version in Menachot 36 found in the majority of the medieval authorities, with the reading ‘and it became dark and he put on tefillin,’ there is no decree at all against putting them on at night; rather, it is only that this is not taught publicly as a practical ruling, and the scrupulous person may put them on even from the outset, as stated in Nimukei Yosef. Therefore he may say ‘and commanded us,’ because a rabbinic prohibition that prevents a blessing is only when they prohibited it for everyone, as with the shofar and lulav, but here there is no actual prohibition.”