The black marks on the bottom are common in cooler climates... The soft / cold water waxes are much tackier, and stick onto your hands/wettie, etc, and slowly migrate to the other side of the board, particularly where you carry the board under your arm... You end up with wax under your wet suit arms, and wax residue ends up on the bottom of the board.
So basically, it is most likely just dirty old wax and wax residue... Very easy to get off. Warm water and a clean rag should get the bulk of it off... Maybe add a tiny bit of dish washing detergent for the final wipe. Rinse well, so the board is not slippery!
My two boards above aren't old enough for any wax residue!

The one on the left hasn't even been waxed in those photos!