Best way to clean engine block top for head gasket?

[RichNJKubota]

Don't forget to clean out the head bolt holes in the block.
I use a scraper and brake clean on the block. If you use some abrasive then stuff a rag down each cylinder.

 

[KYErik]

Thanks guys for the advice - I cleaned it up a bit more with the razor blade and carb cleaner, while being careful not to gouge the surface.

It makes sense that some of the gray/black material is oxidized aluminum- so I didn't worry about that stuff.