Repeating the statements of all of the above -- yeah, you're probably getting the better end of the deal, but Castillo and Oogie could give the other guy 40 steals and 40 saves, respectively, so it could work out.
The main thing here, though, is that (1) there's no collusion, and (2) you're not tooling on some total rookie who has no clue what he's doing. Even if he is a rookie -- if he says he researched it and it's what he wants to do, that's fine.
Agree with other posters -- blocking trades should be used VERY rarely. In one league I'm in some twit just traded Mike McDougall for Mike Sweeney. I posted something to the board pointing out that probably wasn't the best trade in the world, but I'm sure the guy will learn that for himself over time. Either that or he'll remain a lousy fantasy baseball player.