My headcanon is both that a lot of surface folk have a bit of trouble with sporebread, because it is the sort of color that bread made from grain only gets after the mold gets it...

... and that Underdark natives have to learn to pay attention to molds on surface bread, because it doesn't register as 'makes this potentially toxic'. (A bit like how people who grow up with blue cheese are fine with it, but people who didn't usually have the 'you are eating mold' reaction.)

(Texture helps -- mold is fuzzy. But the color isn't as off-putting.)

I can totally see that.

well. Since Drow are very poison resistent

(I think Duergar are too)

they'd probably be the last to know

Dwarves in general are poison resistant, so...

The Underdark is the Australia of Forgotten Realms. Everything is out to kill you.

It is the land down under (ground).

I always love reading the Drizzt books in the Underdark because of the fun scenes with the races who have to bring a torch.

I wish I were comfortable trying to do a game over voice chat.

"Who needs a torch' is authentic D&D experience, can confirm. especially pre-5th where low-light vision existed (so some races that have darkvision in 5e didn't in 3rd edition)

And infravision was specifically heat based.

Yep. Which did lead to fun details (and I'm sure a lot of arguments if you had the one person who knew more science in the group who wanted to be creative).

but is the bread edible?

My group in college used the distance for a summoning to figure out from what height they could drop a celestial bison on a dragon's head. Weird science or math will always intrude.

Spore bread is perfectly edible.

I've been playing a lot of Avernum, which also has bread made from mushroom flour, and a running gag through the series is how inedible it is

I mean, it's probably not to the taste of people used to grain based bread but I have seen people turn their noses at pumpernickel.

even the people born and raised in the underworld who have never seen the sun hate the taste

Word of source material is that bluecap flour is kind of bland, but perfectly edible.