Tableau Data Blending does a left inner join. It is not an outer join.
Joining in the data source gives the option of changing join types.
Actually, Data Blending does not create row level joins at all -- its a variation on a Left Outer Join, but not identical since row level results are not created; the "joining" happens post-aggregation.
For more info, I might suggest Jonathan's response here: Re: Data blending
You can filter out the nulls using a column from the secondary data source and that gets you an inner join.