Ignoring:
If A ignores B, It's solely for changing the color on A's forum view. there is no change or limit on B's ability to use the forum. B is still able to reply to A's posts. B doesn't know A ignores him. A can still reply to B's posts, but A cannot "+1" B's posts.
Blacklisting:
If A blacklists C, the impact is on both A & C's way to use the forum. A will not see C's posts {on the topic list screen}. C will not be able to reply to A's posts DIRECTLY.
A will not be able to reply to C's posts either, since A already chose not to see them.