IF function logical test is blank

I need an IF function with the logical test being that the cell has no value...

Cell B1 needs to either be blank or Underbilled depending on whether the neighboring cell has a value.
=IF(B2 is blank, "", "Underbilled") How do write the logical statement?
B2 has this formula =IF(SUM($K$90+January!$D$96)>0,$K$90+January!$D$96,"")

That did not work. It always comes back with "Underbilled" even though the cell is blank.

I think I came upon the solution though -

=IF(B2="","","Underbilled")

What do you mean spaces in B?
Spaces in the formula in B?
Or a space between B2 as in B 2.