problem soil and want to lay turf

Hello Im new so please bare with me if Im in the wrong place. I am going to attempt to tackle my front garden which is some 250 square feet ish. Last year we took up some paving slabs that had been there many years only to find they were laid on what looks like coal dust. We have left it for a year, as tbh we didnt know what to do, but now desperately need to lay some turf as its such a mess. The only thing is Im not sure anything will grow on it (the weeds are not having a problem mind). I had planned on rotavating the ground and either laying turf or even putting grass seed down to help with costs as its such a large area. Has anyone come accross this that could off some help / advise. I am not sure if I should rotavate and then put some top soil down or if I need to get rid of this black sooty stuff first

 

How thick is the black stuff? If it's only and inch or two, i'd probably dig it in and hope for the best, but if it's a thick layer i'd probably either try to get rid of the bulk of it (good exercise, cheaper than joining a gym) or dig in a small area, seed it and see what happens before i did the rest.