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If a field is "used" in Tableau (is part of a visualization), it cannot be hidden. When you blended on the field in question, it essentially is "used" - therefore it can't be hidden.
This is by design as we don't want to confuse users of a report who see a field being utilized in a report, but then can't actually FIND it to do something new with it.
Thanks Russell for the explanation. I understand your explanation as to why a field is used up. When I do a join, I am using the IDs as well, but I can hide them. I see this as an inconsistency between 2 ways of doing the same thing (blending and joining) as I am essentially using blending different sources to work around the limitations of not being able to run multiple custom queries and bring them into Tableau as different tables in a single data source.
While JOINing and blending effectively do the same thing, they are different. Blending happens inside Tableau (we "use" fields for the blend) while JOINing happens in the RDMBS - Therefore we (Tableau) aren't "using" them to associate different tables - the database is.