Is there a way for a formula to return a true blank?

I'm trying to do a query, for either DCountA or SumProduct, that refers to
another cell for the criteria. I wish to have that criteria ignored if it is
blank.

=If(Isblank(j4),"",j4) -- doesn't do the job because "" actually returns an
empty string rather than a true blank. "" is not treated the same as a true
blank in criteria ranges.

The reason I'm trying to do this is that I'm trying to do several (actually
24) queries based upon selections from dropdown lists.