I would go for getting it chemicaly blacked/blued as you suggest.The cylinder end cap is some kind of alloy and not sure how you can get round this-maybe anodising.I have done this to a mk2 mercury-years ago and it looked great.Also had a mate who had his airporter blued after stripping all the paint off,this also looked fantastic.I think the mercury 's' model was blued and not painted.Go for it!!
Cheers,Pat.