New BP has become lethargic, has black gunk in nose

[Bogertophis]

BPs are more active at night when we aren't looking, especially when hungry. I'm wondering if he has managed to push some substrate into his nostrils?

That could cause some noisy breathing, as can some old skin left in nostrils from previous sheds. But we aren't seeing him & I don't know what to tell you
about scales that are edged in black. It also occurs to me that "old or digested blood" can appear to be black too, so you definitely need to get him checked
by a qualified & experienced herp vet, especially with his change in activity too. Don't forget to lay all these symptoms out for the vet you see, they need ALL
the help you can give them to accurately diagnose what's going on. Tilting his head up to breathe suggests that he is having trouble breathing, and since
snakes cannot cough, any secretions that aren't normal & swallowed can cause problems...it sounds like that's what you're seeing. The "squeaks" you heard
might well have come from "the other end" though, when he defecated; snakes can also pass gas, fyi. If he is no longer "squeaking" it was probably just
the stool/urates that he passed.

One thing, don't take your thermostat's "word for it"- use a digital temp. gun to double check the actual temps IN the cage, ok? (if you aren't already doing so)

Oh yeah, and

Good luck & please do keep us posted on what happens with your BP, & with the vet visit...sure hope it's "nothing" or at least, nothing serious.