I can only speak as someone who owned an 83 K5 Blazer, 6.2 V8 diesel, so not exactly the same beast - well, not at all actually :)
I found that getting standard service item was as easy and cheap as any UK vehicle, I used US Automotive & their service was excellent. Lots of American parts and service suppliers advertise in Classic American Magazine. For bigger bits & upgrades you are talking US - but the strong pound goes a long way to compensate for shipping charges.
If you're talking about a newish Blazer, personally I'd rather go for a Dodge Durrango. My sister owns one (in Atlanta) and we hired one for our last US holiday - superb vehicle (then again, it was the 5.7 Hemi Magnum :D )