ISBLANK not working/malfunctioning

[Excel Chimp]

I'm going mad trying to figure out the issue. ISBLANK is returning FALSE for a seemingly blank cell. The formula is:
ISBLANK(VLOOKUP($A13,data1,COLUMN(C:C),TRUE)

I thought it has something to do with the VLOOKUP returned value. However,when I press delete on cell A13, the problem is fixed. Strange?!
Running just the isblank function on cell A13 indicates a FALSE value. I just can't figure out what is in the cell or how to remove it.

Please give me your advice.

 

[Jonmo1]

Is the best way to locate the null cells (proper term?) some combination of the
Formulas mentioned above? Like an IF=(And(istext).

I usually just use
=IF(A1="","It's blank/null","It's not blank/null")

 

[SamParsons]

That's nice Jonmo, I like it. I had 2 cells I was comparing so I went with Len(A1)+len(A2)<3 as my logical test as the cells would have had well over 3 characters in them had they had a useful value in them (aka not a null string).