What warms more: open pool or covered with a black tarp?

I have a 12' x 30" Easy Set pool. I keep it covered almost all the time the kids aren't in it with a black, opaque tarp that is held down snuggly with bungee cords and covers both the top and most of the sides of the pool. My question is: On a hot, sunny day when the water is cold due to previous weather, is the pool likely to gain more heat while open or while covered by this tarp?

I know there are other active and passive heating solutions. I'm just asking if anyone knows the heating...

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