Selenium toning is a process developed in the 1800s to tint prints in the darkroom. Wikipedia says “Selenium toning is a popular archival toning process, converting metallic silver to silver selenide. In a diluted toning solution, selenium toning gives a red-brown tone, while a strong solution gives a purple-brown tone.”
This image of Laurel Hervert was taken in Tucson, Arizona during the second annual Kelby Photowalk which I used to lead. The bluish selenium tone was added using Nik Silver Efex Pro 2:
This is perhaps my favorite of all the toning possibilities out there. As you can see, it is definitely not red-brown, but has more of the purple-brown hues.