if they can't make domestic links to Gatwick from MAN and NCL with their large catchement areas then how is BLK - LGW possibly going to work.
Good question - but I think the distinction is the lack of competition from BLK.
MAN has flights to:
LCY (VLM)
LGW (BA)
LHR (BA, BMI)
STN (Eastern)
BLK has no flights to any London airport.
Air has a larger journey time advantage over rail for London-Blackpool (rail 3h 10 mins with a change) than London-Manchester (rail 2h 10mins).
Definitely worth thinking about (though it clearly isn't going to happen with respect to the slots being discussed here).