Can you breed for dragons that aren’t unlocked

Yes, the breeding time is always the same as hatching time for all dragons, rare dragons included. This means you can check your breeding time to know what dragon you're going to get; you're getting a Tree Dragon in this case.

You can verify the hatch/breeding time for any dragon with the Wiki's Dragon Reference Chart to check which kind of dragon you might be expecting. Alternately you can just look in the store in-game. Look for dragons matching some/all of the elements of the Parents (combined) that have the same hatch time as your remaining Breeding time.

Note that the upgraded breeding cave/island reduce your breeding time by 20%, but the incubation time will remain the same.

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).