we removed old carpet in the living room, hallway and found vinyl flooring which we removed since it was loose over a concrete slab. Now we have black residue on the concrete. I understand that this
probably has asbestos in it. can I paint over it with a paint specific for
concrete until we decide which glue down engineered flooring we'll put
down or is it better to use the ardex sd f feather finish. Any suggestions
appreciated.

Ardex!!!!!!
Read and understand completely the directions for Cut Back encapsulation.

Paint is going to hinder any bond with the adhesive, later.

thanks so much for your quick reply...was very close to just painting it
and then found this website

In truth it is probably the old vinyl you just took out that had the asbestos. Keep in mind that up to 1984 most vinyl had asbestos in it. When you tear it up to take it out, you make the fibers airbourne......and dangerous!

Before tearing up any vinyl, you should call the manufacturer.
If nobody can tell you how old it is, call HAZMAT!