Blackmon's career line outside of Coors is pretty bad. .241/.283/.370/.653 in 236 games, 878 plate appearances ... rather large sample.
Moving to the AL and out of Coors Field .... I would expect his numbers to be even worse.
Trading away starting pitching depth for mediocre players is not something I would advise ... especially since you guys don't really have a super threatening rotation as is. Now imagine if an injury occurs at the top.
Dickerson is the better player / prospect ... Blackmon would likely be a nobody outside of Coors, but even Dickerson outside of Coors leaves a lot to be desired (though its a smaller sample).
Just look at Dickerson's "great" 2014 ... look how much of that is fueled by strong numbers at Coors Field.