A recurring problem with the blue Transit remotes is they go flat and stop working the remote central locking. This is because the OEM remotes have a rechargeable battery soldered to the circuit board. The remote battery is charged by the coil around the ignition barrel. Like all rechargeable batteries they will eventually fail.
If your Transit has a bulkhead behind the drivers seat you may be unable to access the rear compartment of the vehicle. Only a remote will open the rear.
If you only have one working blue remote it’s a ticking clock until it fails and locks you out. Aftermarket remotes are available which enable you to slot a new battery in whenever you like. This gives you peace of mind that if the battery does ever go flat, it’ll take you 20 seconds to put a new one in. This type of inexpensive battery is commonly available.
Aftermarket remote for Ford Transit Mk7 with user replaceable battery.
These blue remotes are electronically completely different from the similar shaped black remotes. They black and blue remotes are not compatible.