Check out my pressure plate

Crap, I have to go against the grain here.
Even though people have had worse and used them without any apparent problems those black/blued spots mean the metal in those spots got hot enough to surface harden.
Now those spots will wear slower than the rest of the surface causing premature disc wear and in some cases even chattering on engagement.

This isn't just a guess or an opinion by the way.:D

Now that said, if I gave the plate surface a quick surface sanding and those marks came out for the most part I would feel a bit different about it, but I'd bet they don't.