https://dic.pixiv.net/a/白牌
https://www.excite.co.jp/news/article/Goorank_846/
"もともと麻雀の白牌が白板（パイパン）と呼ばれていたことで、そのまっさらな状態が似ていることから生まれた言葉だと言われています。"
4 hits on ndl 
https://ndlonline.ndl.go.jp/#!/search?lang=ja_JP&keyword=白牌&page=1&sort_issued=asc&maintain=true&searchCode=SORT

this also has a dated meaning where it refers to a type of Chinese playing cards, but I'm unclear on which exact cards and how it 
would have been pronounced. In old tax documents on the Japanese playing card tax, there's a category of cards which is "四色牌、白牌
及び紙製ドミノ". (in documents from the 1940's and probably earlier as well) It probably refers to money-suited cards, but there's 
many different kinds...
It's also used to refer to the white suit of Chinese color cards here: http://fukuchigames.blog43.fc2.com/blog-entry-263.html, but 
that's prob a little too obscure to include.