Blackholing refers to the forwarding of frames to a PE that is no longer carrying the designated forwarder. This could happen for traffic from:
Core PE participating in the main VPLS
Customer Edge devices (CEs)
Access PEs (pseudowires between them and the MH PEs are associated with MH sites)
Changes in DF election results or MH site status must be detected by all of the above network elements to provide for Blackhole Avoidance.