Every now and then I drain my block (1999 502 MPI, open cooling system) and the water from the block comes out black. Can anyone tell me if this is normal or is something bad going on?

sounds normal .block,manifolds& risers,thermostat housin should be checked and replaced if reqd , and flush your engine with fresh water more regular

Yup, sounds normal. Try to wipe it up if you can, after it dries you will have a rust stain.

Thanks guys. There's at least one less thing I need to worry about.

I think I will flush my block more often. The problem with that is that I can't find a garden hose flushing adapter for the Bravo 1 low water pickup.