I don't have any experience with that particular product, but I have used other cold blue treatments.
I found that if you try to use cold blue cream as directed, it ends up patchy.
I dissolve the cream in boiling water and use it as a bath instead. It works great and gives a nice even blue.
I see that blue wonder is a two part thing, so I don't know if this advice is applicable.
I have also found that cold blue often turns brown over time. If this happens, thoroughly degrease with carb cleaner, acetone, or MEK and then boil the part in distilled water to blacken it.