IIRC Blue means the airport has a published Instrument approach, Green is VFR, Circle is a military field, and circle with spikes is non-mil.
If you bought the Jepp pack, they have a great key for every single one of their symbols. I'd suggest absorbing all of what they've got there. There's also a number you can call (should be in the pack there somewhere) where you can ask them the significance of everything on the chart and they will do their best to help you out.
S. Adam