What makes it unfunny (to most nonracist people) is that it requires a cognitive stretch to link a black lightbulb with Obama’s coloring. The connection is just not there unless you force it there.
The reason it’s funny to a racist is because OMG the president is a black man is pretty much playing on a continuous loop in their subconscious mind all the time. When they think of Obama, his blackness is what dominates their thoughts. So when they think of anything dark, reflexively they think of him; he becomes shoehorned into jokes in a way others who look passed color do not.
You don’t have to take my word for it. To me, this is something purely intuitive. Like knowing who is insecure in their status by counting how many names they drop in conversation or how they talk trash about others. People aren’t the best at keeping their thoughts a secret. It shows in lots of ways.