Hello Lukasz
The diagram that you are referring to indicates that port Fa0/17 is a backup port. This is a port role that is available on RSTP (802.1w). It is one of the two types of port roles that exist for the blocking state. So yes, in essence, this port is in a blocking state.
However, keep in mind that since SW3 does not have spanning-tree activated, as mentioned in the lesson, the STP topology has not converged. A root bridge will have all designated ports once the topology has converged. So essentially, STP root bridge selection has not been completed.
All of the above is the case ONLY for VLAN 10. SW1 can still receive BPDUs from SW3 however, none for VLAN 10. Remember that all of these links are trunks, and we can see that both VLAN 1 and 10 are allowed on the trunks.
It is important to note that if STP is disabled on SW3, the switch wouldn’t function as being transparent for STP. It would create two layer 2 loops as shown in black and red lines in the following diagram (it was easier to draw than to explain…):
I hope this has been helpful!
Laz