I would scrape out the black carefully. Then spray with a fungicide. When it dries I would seal it over with brown tub and tile sealant using a flat stick, a putty knife or a foam brush to smooth over. You can also use common household spackle, water based paint, or pruning spray or goop. You can just do nothing but I think sealed over looks better and prevents insects from entering or fungus from starting where water holds. I like the sealant because it is flexible and stretches as the plant grows. The brown blends well. Pruning spray is black and cracks later. Spackle is white and stands out. Water based paint is cheap and you can match the color but it does not stretch so it needs to be repainted when it cracks. Dap makes the brand I use and it is in good hardware stores. I use a caulking gun and a foam brush. I squirt it on with the left hand and smooth it with the right and move on.
Quick and easy.