Don't use Google Sheets' ISBLANK()

If you've tried to make formulas in Google Sheets using the ISBLANK() function, you probably have noticed its wildly unintuitive behavior: it does not consider "" as blank. No, that's not two visible quotation marks – that's the presence of the string delimiter. So cells with the contents (omitting the initial " used to display the formulas)

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Despite this somehow charmingly condescending attempt to explain, you're correct in identifying this behavior as wrong. It's the wrong thing to do; it's bad design.

To mitigate this problem, use EQ(LEN(A1), 0), which does what you expect.