How to reference column of DAX variable?
In DAX you can't use a variable name as with tables that are defined in the model.
What you can do is to use the original table names using the table variable as a filter.
For instance like in the following code
DEFINE
    VAR tv1 =
        FILTER( Tab1, Tab1[F1] = &quot;A&quot; )
EVALUATE
ROW( &quot;a&quot;, CALCULATE( COUNTBLANK( Tab1[Value] ), tv1 ) )