The spanning-tree message isn't pertinent. Whenever an interface goes down, spanning-tree sets the status to BLOCKING just as a way to mark that it isn't active anymore. When the port comes back up it will go through the appropriate transitioning states. If it is disabled, then the state will transition to DISABLED.
The issue is the link itself. If you did a loopback test locally, that shows the optic and interface are ok. Next are the patch cord to the panel, the run from the panel to the far end, the patch cord on the far end, and the far end optic.
Start with this:
show interfaces diagnostics optic xe-0/1/0
That will give you some rather handy information about the link itself. These are the important bits:
cmyers@lab-ex4200-fan-1.0> show interfaces diagnostics optics xe-0/1/0
Physical interface: xe-0/1/0
Laser bias current : 5.752 mA
Laser output power : 0.5290 mW / -2.77 dBm
Module temperature : 27 degrees C / 81 degrees F
Module voltage : 3.2030 V
Receiver signal average optical power : 0.6015 mW / -2.21 dBm
What you're looking for is the receive signal power. The acceptable value will depend on the optic type (SR vs LR vs ER). At the bottom of the outbout you will see what are considered the thresholds for usability.
Laser rx power high alarm threshold : 1.5849 mW / 2.00 dBm
Laser rx power low alarm threshold : 0.0407 mW / -13.90 dBm
Laser rx power high warning threshold : 0.7943 mW / -1.00 dBm
Laser rx power low warning threshold : 0.1023 mW / -9.90 dBm
If the rx power is not within the threshold ranges then the link will go down. That means you need to start checking cables to be sure they are good: no breaks, clean ends, proper type (multi-mode vs single-mode), plugged in securely, etc.
You may see this:
Receiver signal average optical power : 0.0000 mW / - Inf dBm
That means there is no signal coming in at all.
-Chad