The OED shows examples of "bluebottle" (fly) from about 1700, long before the development of rhyming slang.

Outside of the song, I've never heard of a "blue-tail fly." The "blue-ass fly," occasionally shows up in sayings like, "go like a blue-ass fly," though I know it only through reading.

In the U.S. we usually just call 'em "bluebottles."