I've got each of them and I'm playing them both tonight. While I was reluctant to accept that Nolasco was back to form even after 2 solid starts, there's something to be said for holding the Red Sox to one run on one hit in five innings...
Blackburn makes me a bit more nervous because, while solid, he's also looked very lucky throughout his past several starts and eventually those deep fly balls could start clearing the wall (plus, he's 20 points below his projected BABIP, and that should go up). But I'm under in innings and need help in every pitching category so I'm willing to take the risk until he gets hit harder.
If you're only looking for one start I'd go with Nolasco, but both are decent plays.