If you are blending based on vertex colour or vertex alpha, then you need a plugin to paint vertex colours in Unity. There's a bunch of them on the asset store. If you are using an image-based mask you could try the TexturePaint plugin on the asset store.
I'm running with an image-based mask setup currently since I'm finding that vertex density is limiting my painting ability. Is this the plugin you're referring to as the TexturePaint plugin? Ideally I would want a plugin that could write to a preexisting texture AND display the final output textures (not just the painted…
I personally have never done any painting of textures in Unity but there are lots of Unity paint scripts out there. You'll have to try them out to find one that works for you. Here's another one to try free: https://www.assetstore.unity3d.com/en/#!/content/33506 You need a shader that takes an image to blend between the…