SW1 is root
SW2 receive the BPDU from SW1
SW3 receive the BPDU from SW1
SW2 and SW3 forward the BPDU to each other after modify BPDU.
NOW
SW2 have BPDU from SW1 (superior) and from SW3 (inferior)
SW3 have BPDU from SW1 (superior) and from SW2 (inferior)
but what make BPDU superior or inferior ?? the SW2/SW3 when modify the BPDU add cost to SW1, and as role root-port is the port with lowest cost-path.
cost from SW2/SW3 to SW1 << cost from SW2/SW3 to SW2/SW3 to SW1.
but how BLK is select ?
since SW2 and SW3 exchange BPDU to each other
the SW2(inferior) compare to SW3(inferior) and the port receive best inferior will be BLK and port receive worse will be Designated FWD.
now
what happened in your case
the SW2 dont receive any BPDU anymore from SW1 so it assume that it now root of STP so it start send BPDU (with SW2 as roor bridge ID).
the SW2 send BPDU to SW3 BUT SW3 already receive superior BPDU from SW1 root port. hence the SW3 refuse this new BPDU.
here the SW3 start send BPDU (superior BPDU receive from SW1) to SW2, and the port now change it status from BLK to FWD because ""port receive best inferior will be BLK and port receive worse will be Designated FWD. ""
