In RGB documents, the 'Darken' transparency blend mode doesn't change colours, but only shows a colour if it is darker that the colour underneath it.
In CMYK documents, 'Darken' acts differently. It does the same thing as 'Multiply': it darkens the top colour based on the colour underneath. In CMYK mode, 'Darken' and 'Multiply' seem to do exactly the same thing.
How can I get an object in a CMYK document to do what 'Darken' does in an RGB document - only show if it has a darker value than the colour underneath?
(side question: why does 'Darken' behave like 'Multiply' in CMYK mode? What's the point of having two different blend modes that do the same thing?)
Here's Adobe's official description of Darken. This matches how it behaves in RGB, but not how it behaves in CMYK: