Pictures would help us.

Still suspect aphids. They come in varying sizes and colors.

I'll get pictures. Less than 1mm long, very skinny.

Just a guess, but sometimes onion thrips will move to cabbage crops in the fall. But you wouldn't see egg cases because of their small size. More likely those egg cases are vacated cocoons -- the evidence of earlier work by predatory flies or wasps.

All the creatures in my b sprouts are aphids. The stressed plants always have more than the happy ones.

There are some videos that may be relevant. Google ~ how to remove bugs cooking "brussel sprouts" ~

(Google don't seem to care if we call them brussel sprouts or brussels sprouts.)

The couple of times I tried BS, they were over run by aphids!
The guys that I work with in the organic produce dept at an organic grocery store said that THEY don't even buy the organic BS, cause they are always so over run with aphids!
My first try, I wasn't wearing my glasses out into the garden and didn't see them til someone mentioned them, the second time, I wore my glasses out there every time and sprayed them every other day with a strong spray of water, and they were still too infested for me to stomach!
BS are one thing I have given up on. Nancy

every time I cook Brussels sprouts in my oven I get black flies come into my house. it's crazy where are they coming from,? I live in Florida.

Not surprising that flies are attracted to that smell. I gave up on growing them, thousands of aphids having a nonstop aphid party all late summer into autumn.

We're starting to eat our brussels sprouts and as I was cleaning them I discovered that there are tiny tiny black bugs deep in the sprouts.

Are you certain the black things are "bugs"? I planted BSs for the first time in a long time this year and found tiny black specks all over in the nooks and crannies around the "balls". They turned out to be tiny turds from asst looper type "worms" which were eating on the top leaves while leaving their poop behind. The egg sacks may be from the "loopers"? Just guessing without a pic though.

My granddaughter just picked the most beautiful bunch of brussel Sprouts ever ,.,,as we were washing them and taking that outside leaf's off just inside and inside a sprout cut in half are these tiny black empty egg like clusters ,,...,.a ranch 25 mi away or so has just discovered the same clusters of tiny specks in hers also EVERY ONE HAS THESE,.,,,