As Jan mentioned, it would be useful to know where you've already looked....
For Pennsylvania marriage records of this period, the first best place to look is FamilySearch's "Pennsylvania, County Marriages, 1885-1950"
This collection is microfilm images of the docket books, not just indexes or transcriptions, so there may be information in the image not included in the electronic index.
The collection name is misleading - although all Pennsylvania counties started recording marriages in 1885, the records for most counties in this collection stop well short of 1950, and some are missing completely (such as Philadelphia, although there are Philadelphia indexes only in a separate collection). Most counties are covered through at least about 1910 - the last docket book for Blair County seems to end abt. Sep 1927.
Even though your parents were living in Blair County just after their marriage, they may not have been married there. A better indicator might be where they were living just before they were married (especially your mother - the bride's family usually chose the location of the wedding). If you're not certain, you can check where your grandparents were living in 1930, and hope they didn't move since before the marriage.
Also bear in mind that some couples married somewhere other than where they lived, and may have gotten their license from either place. In some parts of southeastern Pennsylvania, in the late 19th century, it was fashionable to get married in Philadelphia.
If you can't find records in the FamilySearch collection, you can order copies of dockets (license applications) by mail from the county courthouse. Check their website for contact info, and call before you write. Again, you may need to try more than one county if your first guess doesn't pan out. Don't bother with VitalChek - they charge a stiff premium for on-line ordering of documents that can be had for a few dollars via postal mail.
All of the above relates to civil marriage records - given a religious affiliation, church records are another obvious source, although may require a bit more research to track down.