Yes, Blanton would be pitching in more hitter friendly parks in the NL as a Red. But he'd also be pitching in a more pitcher friendly league environment in the NL. The shift into a pitcher friendly league environment would swallow up the park effect shift. It's a pretty nice transition for a pitcher moving from the AL to the NL when suddenly 7-8 percent of his opposing PAs will be other team's pitchers.
Provided Blanton is healthy, all signs point to a 4ish ERA as a Red, unless drastic league run scoring changes occur (in which case all pitcher's ERAs shoot up and Blanton's performance is very similar relative to his peers).
That isn't going to win the Cy Young, but at 200+ innings that's a helluva weapon to have.