Dragons such as Blue Fire, Frostfire, and Sandstorm have opposing elements. When I choose a fire dragon, cold gets greyed out. When I choose an air dragon, earth gets greyed out. So how do I breed these types of opposing element hybrids?
For cases such as the above, where a hybrid dragon includes opposing element types, it is not possible to use basic dragons. Since the basic dragons are opposites, they will not breed, and that is why they get greyed out as options. In order to breed these dragon types, it is necessary to choose at least one hybrid dragon in the pairing. For best chances, pairing a basic with a hybrid will reduce the number of other potential unwanted dragon types that can result. So for example by this logic Fire + Lichen (Cold hybrid) would be a better choice to attempt Blue Fire than Brass (Fire hybrid) + Lichen (Cold hybrid).
You need to breed a pure dragon with a hybrid that has the opposing element in it. For example, I wanted a frostfire dragon, so I bred a cold dragon with an obsidian. Be warned though, since you are breeding with a hybrid, you could end up using the wrong element from your hybrid. For example, my sister also attempted to attain a blue fire dragon by breeding lava with cold, and ended up with a mountain dragon.