I want to use a grayscale image to smoothly blend two shaders together onto one mesh using a mix shader and an image node. How can I convert the data from the image node into data for the mix shader?
You can use the Hue/Saturation node (Shift+A > Color > Hue Saturation). If you set the saturation to zero then the information will be black and white.
You could use the RGB to BW node to do this quicker Shift+A > Converter > RGB to BW)
You can also switch the image to the non-color data type like so:
This for use with bump or normal maps and similar image files that have data other than colors. It can also help when using a standard image in such a way.