There is no experience adjustment to mobs that con blue or higher. Light blue mobs will only give 25%(or 50%, not sure) of the normal value, and green cons will generally give you 0% of the normal value. Remember that what your client reports as light blue does not exactly coincide with what the server considers to be a light blue mob -- sometimes you will get full experience for a mob that cons light blue if it is near the edge of the range. In the same manner some green con mobs will give experience as well.
I'm not sure how the group dynamics work. I can see it working one of two ways when a group kills a mob that is lb to the highest member but blue to the rest of the group. Either the light blue penalty is applied before the split and everyone gets a really small slice of exp, or the penalty is applied after the split and only affects the higher level player.
In
Eqemu, the mob exp is split among the group before the con modifers are applied, but that is non-classic code and it might(and even probably does) work entirely differently on P99.