I can't speak from experience - all I've had is the blue/blue el' cheap-o model cover (lasted 2 years before it delaminated and got tossed).

However - consider this:

Heat loss is your biggest enemy - and this occurs in pools not so much from radiant heat loss - but from evaporation. Any solar cover (or any type of cover to prevent evaporation) will stop the evaporation problem. Just make sure you have the cover on anytime you are trying to retain heat in your pool.

If you want a cover that will help to 'heat' your pool the most effectively, then clear is your best bet, however even clear will reflect some of the UV energy. Blue will block even more, and black even more. It would seem that black would be best - however I think the heat would mostly accumulate on the top of the cover and be lost to the atmosphere versus being absorbed.

Here's what I do:

If the air temperature is above my water temperature, and I'm trying to get the most out of the sun - I uncover my pool. When the air temp is below my pool temp - I cover it. The evaporation rate is Much Much higher when the air temp is below your water temp.