This is an interesting example, because none of these chords are in a direct sense “in bm” (except the bm itself)!
- am has a C (bm-natural: C♯; would make it A)
- dm has an F (bm-natural: F♯; would make it D)
- F has an F and a C
- F♯ has an A♯ (bm-natural: A; would make it f♯m).
The F♯ however is a very idiomatic chord for the key bm, namely it is the dominant, which is almost always played as a major chord (very often as a seventh chord, a dominant seventh). In fact the F♯ chord in the verse is largely responsible for making it clear that the key is in fact bm: a dominant has the strongest key-establishing effect, more specifically, the resolution F♯-bm has a strong finality, it's an authentic cadence.
And that is quite a good primer for the discussion if some other chord may be appropriate: is is appropriate if it fulfills a useful role. In that chorus, I think the roles are mainly concerned with the chromatic line you (presumably) play on the high E-string, namely E-F-F♯, which you use quite neatly in two different harmonisations, once ending in the tonic, the second time in the dominant (which is a very common trope):
[V:T1] "am"[A,EA=ce] "dm"[DAd=f] | "bm"[B,FBdf]2 | "am"[A,EA=ce] "F"[=F,=C=FA=c=f] | "F♯"[F,CFAcf]2
Actually, because it's a rising chromatic line, this would arguably be more natural to read it enharmonically E-E♯-F♯, and in particular in the second round the E♯ makes a lot of sense as the leading tone of a secondary dominant which brings you to the F♯. Such leading tones are the bread and butter of classical music, and still very relevant in pop.
Now the perhaps problematic thing with B7 is that, being a seventh chord, it would also be very likely perceived as a dominant of sorts – but where does it lead? I certainly wouldn't expect it to lead to anything related with bm, nor with am. That may be the reason you where told you can't do that: B7 seems to set up an expectation that is then disappointed.
Now, that kind of thing too can be ok, if it has a useful effect. What B7 implies is that you want to modulate to em (of which it is the dominant). Going to em might actually work very well for a chorus, it kind of opens up the mood wide, revealing that all the verse bm actually wasn't really the tonic, but just the dominant-key!
OTOH, am is the subdominant of em. Dropping from the dominant to the subdominant is a bit of an odd thing to do – it gives an impression of “wait a moment, there's one thing I forgot...”. That's definitely not the feeling I would associate with a chorus section, however it could work well as a pre-chorus bridge. Either way however, you should do something with the harmonic movement. First setting up B7 which seems to go to em, and then immediately following with that am-dm-bm chromatic thing slaps the listener with two very different, unrelated if not outright contradictory ideas. The only thing that could tie it together would be a melody that makes it clear why. But it might instead end up sounding just jarring / unsatisfactory.
Where B7 could work better is in the end of the chorus, because it would continue this idea of using the E-F-F♯ chromatic over different harmonic contexts.
[V:T1] "am"[A,EA=ce] "dm"[DAd=f] | "bm"[B,FBdf]2 | "am"[A,EA=ce] "F"[=F,=C=FA=c=f] | "B⁷"[B,^DABf]2
If you then follow this with the next verse's bm then this too will be a disappointment of dominant expectation, of sorts, but at this point it might make a lot of sense dramaturgically: it's kind of saying “we see the finish like, but no, the battle isn't won yet, here's why...”.