The Diffuse_Color channel is an input to a built-in Diffuse node which takes into account incoming light to the surface.

The Specular_Color channel is also an input to a built-in Specular node, to simulate directly reflected light.

The Alternate_Diffuse channel is a direct input, without any internal light calculations. Whatever you plug into that channel will add to the surface colour without modification. The same is true of the Alternate_Specular and Ambient_Color channels.

You would do better to plug the Blinn node into the Alternate_Specular channel, and switch off Specular_Color and/or Specular_Value. If you use the Specular_Color channel for a Blinn node, you are combining two specular nodes which work quite differently.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)