That is no longer true, though they remain more difficult opponents than their level suggests. Players who are so inclined can kill them for points after completing the Wanted! quest. Each Black Knight killed raises players' White knight rank, which in turn allows the purchase of additional White Knight Equipment from Sir Vyvin. The black knight location within Taverley Dungeon is ideal for gaining these kills.
An easy method of killing them in F2P and P2P 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.
The organization "Kinshra" may be based off the name of God in the monotheistic Vaishnava Hindu faith, which is Krishna, or the Sanskrit word Kṛṣṇa, because they both literally mean black. Krishna, however, is the merciful and virtuous God, whereas the Kinshra are Zamorakians with "dark hearts".
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.
Black Knights are still aggressive when players disguise themselves as fortress guards, even if they hold the same weapon and shield as one of the guards, suggesting they abuse them for their low rank within the Kinshra organisation.
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 have player animations when being idle/being attacked. Their death animation however is still the one used when a player dies before the Evolution of Combat came out.
After the graphical update, there are male and female black knights.