In Google sheets, how does one either return a blank cell or perform a function based on another cells lack of content?

If you want to end up with a truly ‘blank’ value, you can use the expression IFERROR(0/0). This is different from an empty string which is what you get when you use "". A cell with an empty string will not test true using ISBLANK(), but IFERROR(0/0) will.

For example:

=IF(ISBLANK(A1),IFERROR(0/0),"not blank")

How this works: The IFERROR function takes two arguments. If the first argument is a value, IFERROR returns that value. If the first argument returns an error, IFERROR returns its second argument – but if there is no second argument, it returns blank. We use the expression 0/0 to deliberately force an error by dividing by zero, and because there’s no second argument IFERROR returns blank.