I was wondering why the translation for a black cat is 黒猫 and not 黒い猫. It has come to my understanding that 黒い is an adjictive, so that's what should be used when modifying a noun. While 黒 is just a noun.
In short, it's because 黒猫 is an established single word made of two kanji. As you know, a black cat has its own cultural significance in western countries, and this fact is known to Japanese people, too. Someone in the past must have felt it deserved a dedicated single word. It's similar to English words like windmill, airplane, keyboard, etc. 白猫 is also used, but other combinations like 茶猫 or 赤猫 are not common.
黒い猫 means a cat which is black. It's always grammatical to say so, but it may sound a bit clumsy and it may not carry the connotation the word 黒猫 normally has.