As we know, lunar eclipse appear red, if we can see Earth transit from Mars, does it look like a black dot surrounded by red light on the sun?
Earth would look very much like Venus does during a transit: a black dot moving across the face of the sun. The Earth-Mars distance is greater than Venus Earth, so the dot would be slightly smaller.
The atmosphere, which would be brightly visible from the moon during an eclipse by the Earth, would be lost in the glare of the sun. It may be detectable with a sufficiently sensitive spectrometer, but not visually.