What is this blank purple UNO card?
These cards are mentioned in Uno instructions — you can use one if you lose or damage one of the other cards, or you can use it to include custom house rules card into the deck. See reference in manual:

What Are These Extra Cards For?
Blank Card - This is a special card that you may use in one of two ways. If a card from your UNO deck is lost or damaged you may use the Blank Card as a replacement. Example: If you find that your deck is missing one of the yellow 7's you would simply replace it by taking the yellow Blank Card, marking a &quot;7&quot; on it and adding it back into the deck.
If you family has a special UNO rule you prefer to play with you may write that rule on the Blank Cards and add them straight to the deck

It says also that you should have 4 of these; maybe you have lost the other two?

4 Blank Cards in each color [new decks]