Two different things... Blacklock is an outside back and Parra wasn't looking like having much depth of quality outside backs. the Blacklock money was probably used on Grothe (and if you believe rumours, chasing Roberts as well).
Dunley is a dummy half/half and Parra seems to think it has enough of them with Witt, Morris, Dykes, Thorman, Webster, Hopkins, Hollingsworth, Robson and looks like letting a host of others go including Dunley, Green, S.Witt, Irvine.
Let's face it, while Dunley is good, he is unlikely to have been in the starting 17 next season, and for the money he'd be asking, it just wouldn't be worth keeping him.