http://www.preakness.com/news-center/latest-news/security-pre-race-procedures-preakness-day-announced
Well.
If that is to be taken as gospel, a) there is no out of competition testing, which there was for the Derby, and b) while it says blood will be drawn from all starters in stakes, it does not say they will all be tested. As recently as 2008 they only tested two horses on the Preakness day card.