Bitter AG Stout

I made my second AG beer, a BIAB recipe adapted from NB’s Imperial Stout. I didn’t have a hydrometer, so i did it the redneck way and waited til the grain runoff didn’t taste as sweet… :? whatever. I think my OG was around 1080, and after a week in primary I tasted it. It was not visibly fermenting at all, and there was no bubbling.

It tasted considerably more bitter than I thought, malt bitterness from the dark grains, not the hops. Any idea where this comes from, if it will go away, and what I can do to avoid it next time?

My grain bill was 10 lbs pale, 0.3 lbs each of crystal, chocolate, roasted barley and black malt. The bitterness was not noticeable at mashtime.

My guess would be the black malt. A little bit of that goes a long way.