I think 'most' is stretching it somewhat.
i am sure as many black celebs marry other black partners as do white - it just that the media does zero in on mixed partnerships which makes them appear more frequent than they actrually are.
Showbusiness marriages are difficult for any couple for a vast number of reasons - fame = temptation being the most obvious, but i would dispute that most are violent and do not last - again, they are the newsworthy ones. People being happy and together, what ever their ethnicity, does not sell magazines.