To answer your question yes you can request without knowing the exact date. It is possible that they did not marry in Blair County--do not limit yourself. I would check with Centre County for marriage, first; since you have had zero results from the Blair County Genealogical Society; which holds Blair County marriage licenses 1885-1981; the courthouse may just refer you back to them. Or one of the other surrounding counties. Most marriage dockets are indexed by surnames not given name. Good Luck!!