*
* This function Walks only over Block nodes in the graph. Has it's own visited
* flag, so that it can be interleaved with the other walker.
* If a none block is passed, starts at the block this node belongs to.
*
* This function Walks only over Block nodes in the graph. Has it's own visited
* flag, so that it can be interleaved with the other walker.
* If a none block is passed, starts at the block this node belongs to.