What He Throws
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Blackburn has an easily identified fastball-slider-change-curve repertoire. He throws the slider harder than I expected given his fastball velocity, though given the results from LHB he's obviously not using it as a cutter. Blackburn's fastball is pretty mediocre at 90-92 mph with too much vertical movement to call it a proper sinker. His slider doesn't really have the drop or slide to make it a plus pitch and the curve suffers similarly. His change up is no better. He's a legitimate four pitch pitcher, but I wouldn't call any of them better than average. His off-speed stuff probably qualifies as distinctly below average. There are plenty of starters with his fastball with much better K rates.
He's obviously got plenty of options to either side of the plate, but it's against righties that he's able to consistently get outs. For his career, he's allowed a ~.326 wOBA against RHB (MLB average is about .335) and as the stats show, he does this by scrupulously avoiding the free pass. Fewer than 3% of right handed PAs against Blackburn end in walks. Aside from that, there's nothing impressive about his work. He doesn't really suppress home runs despite his decent GB% nor is his BABIP anything special. The platoon advantage really keeps him afloat here.
Obviously, LHB are really tagging him, to the tune of .364 wOBA against. He's still quite good at limiting the free passes, but he simply gives up too many hard hit balls for that to matter. His effectiveness is highly dependent on the righty/lefty split of the particular lineup he's facing and considering the slash line LHB put up, anyone that gets regular playing time and hit left handed should be in Ozzie's lineup. Kotsay might even be defensible here, given Blackburn's weakness and the Sox' distinct lack of lefty bats.
In any case, he's given up 5 runs per 9 for his career and his tRA over the last two seasons lines up well with this. He hasn't been lucky or unlucky; rather he's shown what he's got and barring the development of a real cutter or anything at all that can generate whiffs, he's unlikely to ever be anything more than this. That said, he's still much better than replacement (and that's worth about a run over 6 IP) and the Sox have certainly been shut down by such pitchers in the past. Worse pitchers, even. Clearly, being in a division race was getting in the way of performance. Now that they're out of it, I'm sure Blackburn will get what's coming to him.