Question: what does it mean when someone says the person is negative on block?
A move being positive or negative on block is referring to how fast the move recovers in relation to how fast your opponent gets out of block stun. As an example lets say a move is -3 on block. This means your opponent will recover from block stun 3 frames before your move has recovered from its animation.
Now lets say a different move is +3 on block. That means your move on block will recover 3 frames before your opponent recovers from block stun.
As a general rule, the more positive the advantage on block, the safer the move is. However just because a move is negative on block does not neccesarily mean it is unsafe on block.
Using the first example above with the -3 move. This only means that your opponent gets out of block stun 3 frames before your move recovers. If he does not have a move with less than 3 frames of startup (and few characters do) he still cannot punish your blocked move because you would be able to block before his attack hits since the start up is to long.
so even if a move is negative on block, it can still be safe on block.