Someone who is riding their bicycle in dark/low visibility conditions, without a headlight or taillight. So-called because they are invisible, like a ninja.
The webcomic Yehuda Moon had a comic illustrating a bike ninja on April 14th.
I wasn't expecting to be out this late, so I didn't have my lights with me, and I was a bike ninja for the ride home.
When I crossed the intersection, I was almost hit by a bike ninja! Dude needs to get some lights and look where he's going.