I think its due to Black women being more focused on their educational attainment and professional careers over marriage.
The percentage of never married for white british women is around 43% just to put into context. So same can be said for a great portion of them as well.
Also i looked at other stats available and Black Carribbean UK women had the similar inter-ethnic marriage rates as the men. So they are almost just as likely to marry white British as their male counter parts.
I think this has more to do with culture in so far as Caribbeans are integrated into anglo-sphere of culture and language, so it's not odd for them to marry anglo-speaking whites in higher numbers than other ethnic groupings. Same can be probably be said anglo-phone and franco-phone christian Africans.
When people decide on long term relationships and marriage they don't primarily look for race/ethnicity as some desirability marker, they are actually in most cases looking at practical stuff and look for things like cultural familiarity, commonalities, career parity, finances, shared values or background. That's why stats from dating sites that are more about flings and one night stands and has demographic majority differences in the apps userbase that don't reflect a true representation.