It depends on the context. If you are describing a dragon that is black, you would probably say: 黒いドラゴン (kuroi doragon) or 黒い竜 (kuroi ryuu).
However, if you want to use it as a name, you might want to use 黒龍 (kokuryuu). Incidentally, this is the same kanji the “Black Dragon Society” (an early Yakuza-type group) said their name.
“Black Dragon Sword” would be 黒龍刀 (kokuryuutou)
There are two kanji for “dragon” 竜 and the older version 龍, both pronounced “ryuu.” You can use either one, but the older one looks cooler, I think.
Black Dragon In Japanese
Black Dragon: 黒いドラゴン
Black Dragon Sword: 黒いドラゴンの剣
that is not the old Kanji, it’s the original Chinese character for dragon in Traditional Chinese. The new character is the Japanese made up character.
Black dragon – warui doragon
Black dragon sword – warui doragon no katana