Can the middle node of my Tor circuit detect if I am using a bridge and/or pluggable transport?
With Nyx¹ (formerly tor-arm) you can see the IP, fingerprint and nickname of the Tor relays that form a cirquit. Middle node can see that a bridge is used, but not whether and which pluggable transport is used from the client (Tor-Browser) to the bridge.
¹Or other tools that use Stem interpreter
I don't want to show a productive relay. Hence: Tor Nyx on a hidden service that syncs a blockchain.
No, it can't: because the middle node is contacted by the first node of the chain, you don't have any kind of direct connection to that node and the connection pipe is unified, i.e. if you're a node and you're contacted - you can not even determine what exactly contacts you - only the protocol commands matter