I imported my iPhoto library into Aperture last night and noticed that they were blurry too, at first. If I left the photo on the screen for a bit, it cleared up.
After it was done importing my photos, it was building a library of thumbnails and previews. It took about an hour to do that, during which, if I opened a photo, it was blurry for a few seconds.
Once all of that was done, the blurriness went way. I could double-click on any photo and it'd open clear.