Hi.
A quick tangent before I can respond to your first comment. My turntable is a 25-ish year old Rega Planar 2 with a Blue Point Special EVO3 cartridge. I have switched out the plinth from the stock particleboard core to two 3/4" layers of solid phenolic resin. That made a big difference. That being said, I still think that my turntable is currently the limiting factor and limits my desire to tweak with resonance control. I could doing a self adhesive layer of resonance absorbing material on the underside of shelves at some point, but no desire now. The lower shelves are not directly connected to the posts. There are tabs welded to the legs that are tapped for screws, so the shelves rest on the tips of screws, which allows for additional leveling and decoupling.
So, for your first question... The black core is the phenolic resin (kraft paper with phenolic resin binders). The top layer is the decorative melamine sheet that is bonded to it. In labs that use phenolic resin tops (epoxy resin tops are more common), black is the most common color and you occasionally see grey. I did one project in about 11 years time with a white top layer. This is left over material from that project. I have matching garage countertops:).
For your your other question, it's been too long since I made the change to this rack and too many other changes for me to give an honest assessment of the improvement. I have no desire to change it. Another side note, I tried out a Mofi Studiodeck with the low output version on my cartridge and it was extremely easy to hear the improvement. I was in heaven. I am hoping to be able to afford to buy one of those later this year.
If you can get the phenolic resin tops for cheap, I think you'll be very happy. if they're 1" thick, you'll be better off than me. if they have epoxy resin tops for sale, you could go that route, but they are a lot more difficult to cut - you would need a diamond tip blade. Wear safety goggles, not just glasses. it's not fun to get a shard of phenolic resin removed from your eye.