It should go without saying, but the Diamondbacks did not bring Travis Blackley to Arizona to replace Randy Johnson. Blackley is a southpaw, and historically a starter, but a rotation spot doesn't seem to be in order. The Australian got a split deal, which will pay just over minimum in the majors.
Unfortunately, it needs to be said - Blackley is not Johnson's replacment, no matter what management blathers. At the link, Jim McLennan talks about Blackley being out of options, but a worthwhile flier for almost no cost.
Based on CHONE, Blackley projects to somewhere around replacement level. If I were to bet, he starts the year in a AAA rotation. There is a little bit of data in PITCHf/x, from two non-stellar starts in 2007 as a Giant.
In those games, he appears to only use a four-seam fastball, along with a change-up, cutter and curveball. Before you look at these numbers, remember, he has picked up a couple more MPH since 2007. And this is just two games.
The release point is skewed on the curve - he does release it " more overhand", but not as much as these flight paths indicate.
Blackley, despite lacking velocity, has decent stuff. He's a longshot as far as making any type of significant impact in the majors, though. His track record says the upside is pretty low - he's no longer a top prospect.