Recent polls gave Nader about 2 percent support in the battleground state so the decision was seen as good news for Democrat John Kerry, who is trailing President George W. Bush in some polls.
The Nader campaign said it would file a lawsuit seeking to force Blackwell to reconsider 9,000 signatures ruled invalid.
Meanwhile, the New Mexico Supreme court put Nader's name back on the ballot Tuesday. The court overturned a ruling that Nader was disqualified because his name appeared in other states as a minor party candidate.
Democrats have opposed Nader's ballot access efforts in many states because they fear his campaign will drain votes away from Kerry.
Wisconsin's Supreme Court hears oral arguments Wednesday in an appeal by the Nader campaign of a judge's decision knocking him off the ballot in that state.
As of Tuesday, Nader was confirmed on 35 ballots, according to Ballot Access News. He has the potential to get on seven more ballots before the election.