From the Comprehensive Rules 4.0:
Each minion is normally played to a certain range of sites beginning with the minion's site number. If the minion is played to (or is currently at) a site that has a lower site number, that minion is roaming. The player must pay a roaming penalty by removing an additional two twilight tokens when playing that minion.
Block symbol has no effect on whether a minion is roaming or not.