Main Difference Between ISBLANK And ISNULL in Salesforce. ISBLANK determines if an expression has a value then returns TRUE if it does not. … ISNULL determines if an expression is null (blank) then returns TRUE if it is. If the expression contains a value, then this function returns FALSE.
When dealing with text and lookup fields, a blank value in the field yields `true` for `ISBLANK()` but `false` for `ISNULL()`—even if you set a text field to `null` in apex it is still treated as blank (i.e. not null) in the formula.