Hi! "Black Rose" is the name of a combo base, so they're 'trying' to produce a lion with that base by breeding specific 'combo factor' bases together that when combined through breeding have a low chance of producing that combo base.
It would be their ideal outcome from the breeding, which requires some forethought to at least make sure the right bases are paired together, hence 'trying for'.
You can read more about Combos in general or look up the specific combo factors in the Wiki here under Combo Bases

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