Replace BLANK with text

Assuming that with BLANK, you mean that the cells are empty on the screen: you may have some invisible characters in those fields, such as a space. You can find that out by using Transform-Extract-Length in Power Query, which will replace the content of the cells in your column with the length of whatever is in those cells. If it is anything other than 0, the cells aren't actually empty, they just look that way.

If the cells are not actually empty, you need to figure out what's in them.

If what you see on the screen is the text "BLANK", use the same method, the length should come out as 5. Anything else, same problem, you have some invisible characters in that text.