And Roast Beef speaks in a monotone with little emphasis or intonation, hence the lack of capitals and punctuation except for the period, usually.
I've always sort of wondered what kind of emphasis Beef was putting on sentences that ended with " !," As in "Man I'm so drunk right now !" I figure it's that sort of laconic emphasis, where we'd be getting a full exclamation if Beef cared a little more.
I picture Blister shouting because he is so small and hard to hear (and incorporeal) - hence the caps.
See, I always assumed that Blister's all-caps-ing was the textual representation of Heavenly, psychic, or other such speech as it worked in the world of mortals -- similar to how Discworld's Death TALKS IN ALL CAPS, if you don't mind the brief geeky comparison. But the strips with Beef and Blister interacting in Heaven kind of put the lie to that, since Blister doesn't talk like any of the other ghostly characters. But similarly, Todd is as small as Blister is; why isn't he shouting?