What are rules for completely blank tiles?
I know some sets use a blank tile for a white dragon. The red one could be a red dragon.

This page has a blank white dragon.

And according to this page (link provided by  L. Scott Johnson):


  There are blank tiles in other sets too, including Chinese Bakelite and Bakelite sets. These tiles were always a bit of insurance that a lost tile would not make the set unplayable: one of the other tiles could be adapted. These sets tend to have carved White Dragons, so these tiles usually remain stored in the case until needed.


So it can be spare parts too.