as black as bo's arse:

[lovepig69]

who is bo and how did the saying originate

[andy hughes]

This is an educated guess, rather than actual knowledge - I think it comes from the days of slavery, when all male slaves were known as Boy, or Bo' as the southern accent would have pronounced it. Since slaves were all black, the phrase "As black as the bo's a**e" could have been shortened to this, but that's just my sumising - QM will probably know for sure.

[Ouisch]

Growing up in Detroit, I've heard almost every African-American ethnic expression commonly used, but I've never heard of "black as Bo's arse."  "Black as the Ace of Spades", yes, but no Bo.  So I don't think it has anything to do with the American South (where "bo" is not usually "boy" with a Southern dialect).  I, too, await the wisdom of QM on this subject.

[Quizmonster]

'Bo' was used as an abbreviated form of 'boy' in the American Deep South. All I can add to Andy's answer is that I too suspect it relates to coloured people in general. Other than that, I might suggest Bo Diddley and Bill 'Bojangles' Robinson, both of whom were well-known entertainers in the USA during the 20th century. The phrase could have been a direct reference to either of them as individuals perhaps.

(Thanks for your confidence in my being able to provide a definitive answer, folks, but - sadly - I cannot do so! In fact, I've never even heard the phrase before. My late response was due to my absence from home for a few days.) Cheers