Rust removal ended with black oxide on the metal?
In the absence of oxygen iron reacts with water forming $\ce{Fe(OH)_2}$, which can react with rust forming $\ce{Fe_3O_4}$ via the Schikorr reaction.  See https://en.m.wikipedia.org/wiki/Schikorr_reaction.

You should use a stronger acid, which will dissolve the rust and bring the iron ions into solution.  You will, of course, lose the original metal that rusted instead of recovering it, but you can't make such a recovery by "home remedies" anyway.  With such an aggressive approach rethink soaking overnight.