A blender lord easily cost 450 pts and you rly want him fighting as many turns as possible. A steed will get him there a turn faster. Also the increased speed will allow him to better choose his combats. Also it provides a good armour save for his protection.
If he's with Black knights then they invariably a smallish unit more used as a delivery system.
With the Blood Knights he'll often have MR2 talisman for 2+ magic ward save on the unit. With Flag of Blood keep he's thus well protected from ranged shenanigans and since he's able to choose his combats his 1+ armour saves is usually more than enough.