Started taking psyllium-high fibre supplement, and have started to notice a darkish brown coloured jelly/thick mucus substance in my BM's. Is this normal when taking fibre supplement?

Mucus is quite common in people with IBS.

It's probably just combined with a little bit of your "poop" and changed the shade to slightly brownish.

Honestly, I have this with every bowel movement.

I've been scoped up and down..... Blood tests... Etc. So, know it happens to someone else diagnosed with IBS.