Tableau's data blending feature is a variation of a left join. Tableau issues a query to the primary data source, and a query to the secondary date source, then joins the two data sources on the linking fields while also aggregating fields from the secondary data source. The * you are seeing indicate that given the linking fields (the ones with the red/orange chain links in the Dimensions window), Tableau is returning multiple values for the chosen dimension. The most common reason I've seen for that is that the aggregation is happening at an insufficient level of detail, so probably more linking fields need to be added to the view.
As far as the second half of your message, it seems like you are trying to link source "a" to source "c" via source "b" using blending and I don't know any dynamic way to do that using blends. You can simultaneously blend "a" to "b", "a" to "c", "a" to "d", etc., but there is always one primary data source in a view that all blends occur off of. You can also use Tableau's primary groups feature to "push" data from a secondary source into the primary, but this is not dynamic and any time you added new values of the desired dimension(s) into the secondary you'd have to update the group.
If at all possible, you're probably going to be better off doing as much of the joins as possible in the Tableau data source (using Multiple Tables or Custom SQL) or creating specific queries in the data source.
Thank Johnathan for taking the time to respond. You're the 1538901398th person that reccomended custom SQL's so maybe that something I have to do/learn. Thank you very much