In respect of the new Blackburn stabling facility, here's a quote from the Lancashire Telegraph.
The project is linked to the £20million upgrade of the line from Clitheroe and Blackburn through Darwen and Bolton to Manchester paving the way for half-hourly services throughout the day from next year.
If it is intended that the Blackburn site be ready for next year then the implication is that it will be ready in time to take over from Blackpool North CMD. However because the Blackburn site is not going to be quite so large as Blackpool North, that makes sense of the temporary facility at Leyland, i.e. to make up the difference.
Everyone seems to be leaving things a bit late, but I suppose that if the stabling sidings (at Blackburn) are usable then everything else (carriage wash, etc) can be added later.
I don't think there's any reason to suppose that because the refurbished Blackpool site will be able to accommodate electrics that it will necessarily be barred to diesels. Lots of depots/stabling points are fully electrified (Preston, Longsight, Stockport, Edge Hill) but they still accommodate diesels. I wouldn't be surprised if the four units off York-Blackpool services continue to stable at Blackpool, perhaps even a couple of Colne units as well.