Your cell probably contains an empty text string (""), which will return 1 with both functions. Depending on what you have in the other cells there are ways to ignore that. Please post a sample sheet with some data and manually mocked up results if needed
To check if A1, say, has an empty text string, type =TYPE(A1) somewhere. If the result is 2, it is text.
Now, how did it get there ?

__________________
Using O365 v2503 - Did you know you can thank someone who helped you? Click on the tiny scale in the right upper hand corner of your helper's post

Type(XXX) does return 2. Very helpful, thank you.
There are lots of other TEXT cells on the worksheet but they all contain text, so they return 0 for the COUNTBLANK function.
That gives me enough clues to write a procedure to identify any cells with the empty text string. I'll do that in VBA as it is a very large worksheet.
I too wonder how these empty strings crop up from time to time - not often though.
Thanks again
Martin