Pretty simple. The same thing as the Transparency property that Texture and Decal classes already have. As a use case: Recently I’ve wanted to make a Place with the Metal Material and the CorrodedMetal Material; however, the transition from certain Materials, like in my case, (Metal to CorrodedMetal) is very harsh. To achieve this currently, I have use a technique which layers two Parts inside of each other, one with SmoothPlastic, and one with slight Transparency and the Material of your choosing (ex. (SmoothPlastic) Part1.Material.Transparency = 0, (CorrodedMetal) Part2.Material.Transparency = 0.5) to make it less harsh. This is a different use of the technique, but still the same thing: http://devforum.roblox.com/t/material-mixing/17663 He calls this “Material Mixing,” which would be another useful feature that would go well with this request. As you can imagine, if this technique is used frequently, it could really add a number to the polygon count. This would be useful for Materials that dramatically change the look or color of a Part. Also, StealthKing95’s post below.
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If I may expand on this, texture blending would be amazing! I could make for example a brick wall have areas where the bricks are especially damaged or worn out, patches of dead grass in grass, and all that jazz! It is basically your request but even more capable 
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For example, here a brick wall texture was blended with a plaster texture
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Feature Requests > Client Features
(since I’m that guy)
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