Most Helpful Guy
If you told him you unblocked him it sends the signal that you have feelings for him.1
Most Helpful Girl
It can imply you're ready to move forward, with no angry or bitter feelings or it can yes, be taken as a sign of you being curious about him or wanting to keep a door open.
But, if he's your ex, and you have moved on, then it shouldn't matter what he thinks.
I keep my last ex blocked just because I really have no desire to ever talk to him again :P0