I was playing Uno then someone played a yellow blank onto a yellow 7, is this allowed?
The blank cards in UNO can serve a couple purposes. Their main purpose is to serve as a replacement for lost cards, just write the number on it and your good to go.
Alternatively, they can be used for house rules. So long as they made it clear that the card was wild it would be valid. The rules suggest writing any house rule on the card to make it clear what it may be used for.