Dunno about the black-specific "echo" words, but some of them originate elsewhere.

"Couscous" is of African origin, but *not* black African. North African people are of a similar skin colour to the Greeks, Turks, and French - swarthy but not black by any means.

"Bebe" is French (for "baby", naturally). It should have acute accents on the "e"s, but I can't be arsed to find how to do it on a UK keyboard. :-)

"Mimi" is a name on its own, of French/Italian origin.

And BB, CC, DD, etc - all originating from names (or nicknames) which did have those two initials. I'll bet there's people out there who's called their kid Owjay or Ohjay since Orenthal James Simpson's abbreviated name became widely used.

I think the rest of your echo words though derive fairly clearly from how adults speak to very young children. "Look at the bow-wow." "Bunny-wunny." "Incy-wincy spider." Etc, etc.

Re "Hilo", it's also the Spanish for "thread" (according to Babelfish). Given the cotton trade, I suspect the town got named after its major product.

Graham.