What would cause the water in a swimming pool to be blue not clear?

You need to add chlorine.

I doubt in the above answer that chlorine will clear up a "blue water" situation. BTW, is the water or the pool surface - floor, walls etc - blue - there is a huge difference. The blue affect on the plaster could mean a greatly out of balance chemistry - pH and Total Akalinity.

Never heard of the water turning blue unless there was an additive put into the pool that would purposely turn the water blue.

If you're asking about the pale blue colour of swimming pool water, it's because water absorbs light. Light that is visible to humans comprises the seven colours of the spectrum. The light at the red end of the spectrum gets absorbed, but the blue does not, hence you can see it in water of sufficient volume.