This bird seems solid black with a long tail. I don't see many solid black birds in ebird in Zimbabwe. My closest guess is a Southern Black-Flycatcher, but not sure.
This is the same picture, one is just lightened up a bit. Usually I like sunny days to bring out the color in the pictures, but I think if it was a cloudy day, it might of helped with the ID.
https://forums.whatbird.com/index.php?/gallery/image/796-zimbabwe-black-bird-long-tail-pic-1-light-copypng/
https://forums.whatbird.com/index.php?/gallery/image/795-zimbabwe-black-bird-long-tail-pic-1-copypng/