The darker stem looks like it is rotting. Sometimes it just happens but check to be sure you aren't keeping the soil too wet. It should be moist but not sopping. Err on the side of being a bit dry if you're not sure.

You mentioned you took the cuttings a week ago and they're getting partial sunlight 7 hours a day. I would suggest no sunlight at this stage - just bright indirect light. Your cuttings are working hard to develop a root system and survive; the sunlight just adds additional stress at this point.