Hi
Switch2BLK means it will only transmit BPDU and will only receive BPDU, ( actually it can receive whatever traffic but all traffic will be dropped, exept for the BPDU:s).
Switch1FWD means it will forward all traffic that is normally flooded,like broadcast, maybe multicast depending on configurations, multicast in 224.0.0.0-255 range, and unknown unicast destination.
If you see other traffic like ICMP ping replies the source and destination mac-address should be known. Look in switch1 to see what mac-address-table looks like for the traffic you traced
/Mikael