Black Knights are medium level humans found mainly in and around their fortress. They are some of the oldest non-player characters in RuneScape. When the game was first released, Black Knights were the strongest foes in RuneScape at level 46. There were only three Black Knights in RuneScape at that time.
An easy method of killing them is to kill them near the Black Knights' Fortress, using a fast and high-hitting weapon, and good melee armour, or use magic and take cover at one of the safespots. When at low life points, you can go to the Monastery and be healed by the monks, then go back. If you have 43 prayer you can also use protect from melee when at low life points, and heal and recharge at the Monastery.
Ironically in Armies Of Gielinor they appear on Zaros' side as some of his units, this suggests the black knights may have once served Zaros and split up. Some remaining loyal to Zaros and some joining Zamorak.
In RuneScape, Black Knights use black longswords and have red plumes. However, in the RuneScape-based FunOrb game 'Armies of Gielinor', they wield black two handed swords and have different coloured plumes. In Armies of Gielinor, their boots seem to be black snakeskin boots.
Black Knights used to drop a pile of flour on the ground. This could only be picked up if a player had a pot in their inventory, resulting in the usual Pot of Flour.
The longsword graphic update has changed the colouring on their swords to look like the new Black Longswords.
Black Knights are among the few NPCs to have more than one capital letter in their name. If Jagex had followed their common name structure, "Black Knight" would look like "Black knight".
The Black Knight in Port Sarim jail has a lowercased k in 'knight.' He is also the only black knight not wielding a sword.
It is said that in the RuneScape Official Handbook, Black Knights weigh over 300 pounds and also says that the 300 pounds is almost all muscle. This is most likely hyperbole.
All Black Knights and Elite Black Knights received a graphical update when the quest "The Death of Chivalry" was released.
Along with the aforementioned update, Black Knights use the updated animations that were released with the Evolution of Combat, including entering a combat stance when under attack. They still use the pre-EoC (and current Legacy) death animation, however.
After the graphical update, there are now male and female black knights.