"A cat is black."
Translation:Un chat est noir.
In english (and in portuguese), the phrase "A cat is black" is too vague, has no information, as it is telling something about some cat in the world (wich?), not about a specific cat. ("The cat is black" is better). Is "Un chat est noir" natural in french? Does it have the same problem?
I believe when it is an animal you can tell by what gender you are given for the animal. If it says chatte then use une, if it's chat then use un. The same principle applies to all animals, if you have the female animal use une and use un for the male animal.
If you are unsure then stick with the masculine.