Generally, I think it is better to write:
=IF(B5="", "", B5*$C$1 + B5)
Notice that the formula above (suppose it is in C5) might return the null string (""). So the cell might appears blank. But =ISBLANK(C5) would return FALSE (!).
The reason is: ISBLANK is a misnomer. Think of it as ISEMPTY. A cell is "empty" when it has no value -- no constant and no formula.
C5 is not "empty"; it contains a formula. But it appears blank because its value is the null string.
In contrast, =C5="" returns TRUE if C5 is "empty" or (key point) its value is the null string. So =C5="" is TRUE if the cell appears blank.
And that is usually what we want when ask if a cell is "blank".
Arguably, there are times when we truly want to know if a cell is "empty". Of course, use ISBLANK for that purpose, and only for that purpose.
Also, there are other cell values that apppear blank -- a string of spaces and/or nonbreaking spaces, for example.
Instead of trying to handle all values of cells that appear blank, it would be prudent to be consistent and always use the null string when you want a cell value that should appear blank.
