I've found that going back into the soda after tea toning I get nice neutral greys. With heavy exposures and lots of time in the tea I have had jet black shadows but by then the paper is quite stained by the tea.
Back to the original question add classic cars and bikes to the list of stuff that works in blue. I am especially fond of a cyanotype I made of details of a Haley engine.

Who was it that said anyone who prints landscapes in Cyanotype is a vandal??