I've never noticed two-word phrases at the top of this setter's grids, though to be fair, I've never looked for them. If that's correct, 1a must be "bLeak".
4a Buchan novel's dark revolutionary with place underground (9)
The clue for 4 across is - Buchan novel's dark revolutionary with place underground (9)
I know the answer but don't want to post it for fear of spoiling it for others, but in my opinion, it means that the answer to 1 across is the alternative answer.