The transmission is not scynchronized so it"s difficult to shift "on the go". It may have normal wear because it"s over 50 years old. It"s also possible the clutch needs adjustment because it may not be releasing all the way.
The three point"s connection to the transmission is that the transmission drives the pto and also drives the hydraulic pump that powers the three point. the only connection between the blade and the transmission is that the blade is supported by the hydraulics which is driven by the tansmission via the pto shaft. So I can"t imagine how the blade would make a difference.
Our TO-30, if left in gear on an incline with the engine off, will be difficult to shift to the start position. That"s because the driveline is continiuously connected to the transmission ( that the reason you need an ORC for pto driven attachments with high rotating inertia) and there is a load on the drive line caused by the weight of the tractor and the incline. Now if you did that with a heavy blade on the three point, the load would be greater and it would more difficult to shift to start or any other gear. In my opinion, this is a normal condition for a 50+ year old Ferguson.