With a blue cover, you will probably get more heating with the cover off because the sun's rays can reach the bottom of the pool and heat from the bottom up. With the cover on you are transferring the heat from the cover to the top few inches so you will still get some heat that way. The tradeoff is that it's a pain to keep pulling the cover on and off. If you know it will be mostly sunny then that would be a good day to take the cover off. A solar mat might be something to consider - it's a big black mat that hooks up to your filter so water is pumped through it and the sun warms it up before it goes back in the pool.

Peter