Rust diseases have a strong orange color (which is why they are called rust). And blight is typically grey and then dark brown.

Rust is often on the underside of the leaf and can be raised up from the surface.

Also, rust disease produces many mobile spores . that blow on the wind. So, if you rub your finger across the diseased area you should see spores on your finger. Blight are water molds so only transmit via water splashes. The spores dont rub off in the same way

But note that in some cases blight can be caused by bacteria

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