If I understood your problem correctly, you have a female neon blue dwarf and a male flame dwarf and you're worried about them hybridizing?
... which isn't actualy possible as they are both the same species so cannot technicaly hybridize in the first place

It's more like crossing a blue budgerigar with a green one or a red guppy with a half-black guppy. It's perfectly fine to do. Also, don't expect any exciting mixed up fry from this sort of cross. Because of how genetics work, the fry will probably be the natural striped wild color - not a mixture of the two color morphs. Also, if I interpreted your worries correctly

, I'd like to say that the chances of your actualy getting mature fish if the pair spawns in their tank are pretty much non-existant. You'll need to plan it and set up a breeding tank with a sponge filter, heater, tight-fitting hood, lights etc and get a culture of microworms and infusoria or some other substitute minute foods going before you can start thinking about that.
Having said that, maybe that's not what you meant and you're concerned about something else? I don't know. The only other thing I can think of to worry about is that the male, both because of the very fact that he is male and because he was in the tank first, will be quite aggressive to the (apparently?) newcomer female and may also chase her around a bit later on if there aren't enough hiding places or if the tank is too small. Idealy, the tank should be at least 20 gallons and I'd probably add another female (re-arange the tank beforehand) to spread out the aggression a little more evenly as well. Besides these things, be aware that these fragile fish often carry disease. I would quarantine new additions in future, to be on the safe side, for at least a month.