Blend modes in Unity
A really cool article by Artyom Sovetnikov about implementing blend modes like overlay, multiply, color dodge etc. in Unity.
Check out the article on Artyom's blog, here:
Included is a project showcasing the different implementations covered in the article. It's available on Artyom's github, here:
Darken is the only blend mode included in the above project, but examples of all kinds of different blend modes are listed in the article so it should be easy to create a shader implementing the one you need.
Artyom also has a commercial asset with all the blend modes and additional functionality if you just want something that works straight out of the box: