My understanding is that there is no requirement to be 'completely' inside the tackle box: I know for sure that for Team A, they must only be 'catching a piece' of the tackle to be considered inside the tackle box, so I'd assume the same is true for B.... I know what we all say about assumptions, but I've never seen/heard anything that is more restrictive for B in this regard. Relevant rule is 9-1-6-B.
That said, B must also be stationary, within one yard of the LOS, for any BBW to be legal, and the block must also be in the tackle box, so they can't dive outwards if they are on the edge.