The 98 Blazer comes standard with a fill plug, not a drain plug. Dorman does make a aftermarket cover with a drain plug. If you have the factory cover on your Blazer, you are talking about the fill plug. I don't know the size, but you should be able to pick up a replacement at most salvage yards. The fill plug is not located on the cover.
You say the hole is stripped, so you may be asking so you can tap new threads. I don't know the size.