Either you had uncolonized coffee grounds or your block was contaminated with black mold but those black spots don't look good. You want it to be all white and fuzzy before fruiting.
You don't want to put your block to fruit if there's uncolonized substrate because there's a high that you'll get a contamination.
I'm no expert though, I'm just starting like you. Following and looking foward to seeing your next grow. So glad to have found another mushroom grower on Steemit.