Why's does that surprise you? Christianity is most popular in poor black southern regions. Black people are more likely to be Christians than white people.
Christianity is also much stronger in Africa than in the West.
I think it's possible in the near future Christianity will become a predominantly black 'thing' worldwide.
https://www.pewresearch.org/fact-tank/2018/04/23/black-ameri...
To add my POV, the phrase "A surprising number of..." doesn't always mean "I'm surprised". I use that phrase to highlight note worthy facts:
E.g. The Tacos are surprisingly tasty (note: Tacos are ALWAYS tasty to me)
Not sure if you're joking?
The black evangelical Christian church is a media trope. It's a huge part of many black communities in the US, especially in the south.