I am guessing you might have a snake room with a higher ambient temp?.
I think 75 or 77 would be fine for a blue beauty's ambient. thats what The ratsnake foundation says. You may right your breeder if your worried.
As for me I keep my snakes in my cool living room. Which is Why I have to heat both sides of my Ball pythons cage...she just loves it I'm sure
Eventually when I get my boa and anaconda I'll probably have to do the same.
But my future colubrids will do fine in it.
Ting's low is 70. He's been hanging there all day.
it couldn't hurt to ask your breeder but I think 77 is fine. I am glad to know the open mouth isn't a horrible sign.