Multiple people DID say that clear covers were the best for heating. If you want maximum heating of the pool then you do want a clear well-insulated (such as bubble-type) cover because this lets most of the sunlight into the pool to heat it deeply (i.e. not just at the surface). The downside of such a cover is that if it also lets UV through (not just visible and infrared), then chlorine breaks down as it would if you had no cover.
There are some that promote a black cover for heating, but for this to work at all the black has to be on the underside of the cover where the cover touches the water while the top side of the cover above the insulation layer must be clear. However, while this situation prevents chlorine loss from sunlight, it also ends up heating from the surface of the pool so you need good circulation near the pool surface to distribute the heat throughout the pool. Otherwise, it is not as efficient since the hotter pool surface temperature has heat loss back through the cover.
By the way Mark, the earlier concern of a clear cover letting radiation back out isn't a concern in my opinion because an opaque dark cover would likely absorb that radiation and then that energy would get thermally transferred to the other side of that surface and removed through convection to the air or re-radiation back to the sky. It's only a white or reflective cover to infrared that would retain the radiation in the pool, but as you point out that's a minor source of loss anyway.
