Black lumps embedded in garage concrete flooring?

We had vinyl sheeting on the floor, it wasn't stuck down with adhesive. We just rolled it out and put workbenches etc on top to secure it down. It's been fine for the past 20+ years. I am currently in the process of revamping my garage. I’ve taken up the vinyl sheeting and found these black lumps which look like little lumps of charcoal/tarmac. Some of these lumps are hard, other bits just crumble and turn powdery. It's defo not mould as I've jet washed the whole area, I also noticed they bleed brown when wet, smilier to if you got water on coffee granules. Any suggestions as to what it could be?