The bottom blue crystal is on the Imperial controlled Raxus Prime. After you beet the AT-AT, go over to either side of the room where the other ones are just standing. There is an empty space where an AT-AT would have been, there is a hatch you can jump up into, and then jump up into another room. On one of the sides there will be a blue crystal in a corner.
It is a little hard to get up there, first you will have to jump onto the thing that is hanging down, you should be facing the back of the holding space. Once there jump and hold forward, this puts you on a little platform. Jump again while holding forward and you should be in the room.
Remember to check both sides.