Take a look at this question: https://answers.unrealengine.com/questions/970349/how-to-blend-between-two-materials.html, it might be helpful.
Hi I am trying to lerp between 2 different materials in blueprints is there any way like this. Or i need to do anything new
Here the expected output is lerp between Matrial A and material B
You can’t lerp in blueprints. You need to setup the material with a lerp controlled by a parameter, and then change that parameter from your blueprint…
Actually I am using single master Material C and from that i created 2 instances of Material A and Material B . How can i proceed further step as you said
Nope, sorry 
You need to have one material instance which you can control with parameters, then you can control it with blueprint.
It must be possible to make one material like this. If you do that, I’ll fill you in on the rest.
Or show me your material?..