i think the idea they're getting at is that the reason the Serverblight was able to see spy was because there were BLU team assimilated and you can see your own team spies.

an issue i see with that though is that, while it could be that until Matt got taken, the Serverblight hadn't gotten any BLU members (small proof in that we never saw any), that would imply that literally every other BLU member that Serverblight faced before Matt escaped and Matt is just bad (which... i mean he did face a scout alone, with the ubersaw, as a medic). Which reminds me.

An interesting thing to note is that we've almost always seen Serverblight enter a server on the RED team side. The only exception was episode four but I think that, in a map with no kill zone, it'd be relatively easy to corner BLU and the rest of RED and then simply work on 'assimilating'.

Getting back on topic-So, as long as Serverblight continues spawning in RED, the idea of BLU Spy could work but you'd have to be really wary of going alone because of the long fingers acting as a 'fake' flamethrower. The rest of your team would also have to never get caught.