Trouble is, you can't be sure if your subbies used a solvent, of course they will deny it.
Bitumin is a heavy oil product mixed with clay and water so you have all the elements there to cause a few problems for any product used but usually they are bullet proof it almost certainly will be a solvent based product that has caused the tar stains on top, IF it is tar to remove it you will need a solvent (in the past I used white spirit) applied to a cloth used across the top of the pile only and extracted out almost immediately,even then its doubtful it will all be removed.
Usually though it's just soil wicking back you will need to bring in your extracta and pull it out , you can also use a roto with a dry pad over the extracted area if you want to speed drying.
If you want to know if they have used solvents lift the tile look underneath and in some cases you can see bubbling if it has bubbled the tile is a write off. You can probably lift tiles from other less conspicuous areas and replace/swap the damaged ones with these.