"Blimey," said the other twin. "Are you ---?" (PS6)
"Blimey, I'm tired," yawned Fred, setting down his knife and fork at last. (CS3)
"Blimey," said Mundungus weakly, looking from Mrs. Figg to Harry, and back again. "Blimey, I -" (OP2)
"Blimey...you reckon it was this sign again?" (DH22)
"I can't - blimey!" (DH23)
"Blimey, a baby!" (DH25)
"Blimey, Neville," said Ron, "there's a time and a place for getting a smart mouth (DH29)."
A British slang exclamation, usually used by Ron Weasley.
A term of exclamation, a bit like "wow". It's actually a contraction of "God blind me!" (hence variations: "Cor Blimey!" or "Gawd Blimey!")
It's very much a slang term; someone trying to sound sophisticated/well-educated wouldn't use it (at least, if they knew what they were doing). It would sound very unusual for Percy, Hermione, or McGonagall to use it, and it's difficult to imagine Snape ever saying it.