Was about to make some squash/scallop burritos for the freezer and cut into a nice looking sweet onion...
A blackish, powdery fungus or something. How dangerous and what to do?
If it is between the skin and the first layer or so, just peel the onion down further. Throw away the onion with the black mold on it, rinse the remaining bulb, and then chop and use just like you normally do.
It's a flavor issue, not a health issue.
That's what I'd do, too.
Apparently, molds can sneak long, nearly-invisible tendrils down into soft foods, so if you find mold on top of a jar of jam, say, it's recommended to throw the whole thing out. But on a hard cheese, an onion, etc. you can throw out the moldy bit and eat the rest.
Enjoy your burritos!
P.S. When I saw the subject line I thought this was going to be a new dance you were going to teach us all! I was looking forward to photos.... ;-p
This is aspergillus niger, a very common mold for onions and garlic.
The USDA says you can peel away the affected areas and then use the remainder, but personally I'd never use such onions for canning because there's no way to tell how far invisible spores and threads have actually penetrated.
For fresh use or frozen, it's your call. Health risks are low, though eating infected onions isn't risk-free.