I’m not sure why. I’ve baked this cake by using black sesame powder bought from the store AND by grinding my own. Each time, the interior comes out dark and rich in color.
If you grind your own sesame seeds, they may need to be ground for a longer time. Grind the seeds until they are slightly sticky and release their oil. This takes longer than you think. Look for a powder-like texture that clumps and sticks together.
The outer crust, though, is usually brown. Sometimes, the crust looks more black in some places. But the edges and tops are almost always brown.