Why doesn't the Rambam mention the Brochah on Tevillas Keilim? Even if he holds its a Mitzve Drabanon (as this is subject to an argument), but by others he specifies the Brochah.
So why is it different here?
The Shulchan Aruch brings down the Beracha in YD 120:3. Be’er HaGolah quotes the Tur ibid. and the Mordechai at the end of Avodah Zarah as his source. The Bi’ur HaGra cites the famous teaching that all mitzvos get a beracha beforehand as the rationale.
But it doesn’t seem that the Gemara ever explicitly prescribes a specific beracha for this specific mitzvah. As such, I’d like to suggest that the reason the Rambam doesn’t bring down על טבילת כלי(ם) is because he holds one shouldn’t say it.
This is similar to how, in Hilchos Tefillah ch. 7, he says one should say the various Berachos in Birchas HaShachar but omits הנותן ליעף כח. The Shulchan Aruch concurs that one should not say it (OC 47:6), and the Taz explains that
דאע"פ שנתן הטור טעם לסידורי אשכנז שיש ברכה זאת [...] מ"מ כיון שהברכה ליתא בגמ' ע"כ אין לאומר' וכ"כ רש"ל בתשו' סי' ס"ד
Even though the Tur brings a reason for the Siddur Ashkenazim which has this Beracha [...] nevertheless, since the Beracha is not in the Gemara, therefore we shouldn’t say it. So, too, does the Rashal write in his Responsa §64.
Why does the Shulchan Aruch bring down the Beracha of על טבילת כלי then, if he seems to agree that Berachos not in the Gemara shouldn’t be said? Perhaps he holds that the fact that we make Berachos on doing mitzvos is enough to validate it, while the Rambam would hold that the Beracha has to actually be in the Gemara in order to be said.
I have no source for any of this other than my own logic. That said, if someone has posed this interpretation of the Rambam, I’d love to hear it.