But it is radiating it 'back out' less efficiently than a white roof. Thus black solar panels may indeed be a net negative if every building otherwise would have had a white roof.
In a land of mostly dark roofs it's probably a net positive.
Unless you pump the energy deep in to the ground it will still eventually find its way to radiate back out in the form of infrared blackbody radiation. The timescale we're talking about isn't very long in the grand scheme of things, and the entire surface of the planet is doing this to some extent or another.
Even if we did one of those megasolar projects people like to talk about like paneling the whole Sahara or Mojave, we would still only be absorbing a tiny amount of the petawatts per second the sun is constantly bombarding the planet with, much of it being absorbed by plants, the ocean, darker rocks, etc.
Our solar panels just don't hit the scale that the melting ice caps do, and the only thing we can do to stop that is reduce the flow of new ghgs into the atmosphere.
In a land of mostly dark roofs it's probably a net positive.