I have two blueberry trees, 5-6' tall. Both got lots of flowers and they falling off now for fruit. However, one got some amount of leaf and the other one got very few leaf. The leaf look OK (Green) though.

So what is the problem? Why each tree is different in term of leaf amount? Should I be concerned?

I put the tree about 15-20' away from the pine tree which I think the soil is acidic enough. I haven't tested though but I did put good compost under it and had some fish fertilizer many weeks ago.

http://www.gardenersnet.com/fruit/blueberry.htm
Sounds like your doing it right.
May want to get a soil test.
We use Cypress mulch around them to aid in the acid and keep back the weeds. It seemed to help.

nitrogen will add leaf growth but at the expense of flowers. If you're concerned about the lack of leaves, use a high nitrogen content fertilize around the tree you're concerned with

Just in case you also don't have much fruit?
The majority of blueberry bushes do not self pollinate, and you need two different varieties (not two of the same) to accomplish pollination. We had them once, and learned this and had to plant a third variety to the first two...then we had lots of berries.