I live in Canada, and whatever part of the country I travel to, the Blues scene coukd be described as "a bunch of white guys, playing black music, on Japanese instruments".

Whatever, the Blues is a feeling, one that finds musical expression... and it ain't all gloom'n'doom.

So I think the answer to your question about metal guitarists being able to play the Blues, is YES. Practice gives you the vocabulary to enable you to say whatever it is that you want to say... if other people try to pigeon-hole you as a 'metal guitarist' [u]only[/u], they aren't really your fans. They only want to hear what they want to hear.

Just about every heavy album from the eighties has at least one slow, 'soulful' track on it, almost as if the bands felt a need to prove that they are 'legitimate' musicians.

I think Rainbow would qualify as an 80's metal band. Check out "The Temple of The King". It ain't Blues, but it do have soul. We can't all be SRV, but none of us can only play one kind of music.