MUFC Fan,
Let's get one thing straight, BLK will not decide on the future of a LON connection, the airlines will

. After all, BLK only provide the (not so great) runway to ops from.
Revenue management will no doubt have played some part in FR's choice to leave the STN/BLK route, their asset being far more profitably utilised elsewhere (regardless of the pax figures quoted

).
Don't expect a new J31 operator (or anyone else for that matter!) to jump in and fill the void, their assets have yet to be proved profitable on lesser routes.
Don't get me wrong, I'm sad to see FR depart the route (which, to respond to another poster, was theirs from the start utilizing BUZZ 146 post takeover) however, it's going to take huge balls of steel, careful math and something soft to fall back on, before any sane operator exposes themselves to this, not for new-starts, up-starts or the faint hearted route.
Rgds
The Moss