GrayScale shader with Lightning support and Transparency support, iOS

Hello, in my game I need sprites that I can change colour saturation of. Moreover they have to support lightning and support transparent areas. My work resulted in such a shader (GrayScale.shader [shader_link][1] ) The problem is the line Blend SrcAlpha OneMinusSrcAlpha which prevents shader from working on iOS devices Please help me to replace this line without missing functionality [1]: http://forum.unity3d.com/attachments/grayscale-shader.103237/?temp_hash=ae98c92d4f44dc72b52c89ead41efdbe