If you control "Blade Knight" and a face-down monster on your side of the field, "Blade Knight"’s second effect is not applied because you control another monster (whether it is face-up or face-down is not important).
If you have two "Blade Knights", then their second effects are not applied because for each one, you have another monster on the field besides the "Blade Knight".
If "Blade Knight" destroys a Flip Effect Monster like "Slate Warrior", even that monster’s non-Flip effects are negated. So in this case, "Slate Warrior" does not reduce the ATK and DEF of the "Blade Knight" that destroyed it, because "Blade Knight" negates its effect.
If "Blade Knight" destroys "The Immortal of Thunder", its effect is negated. If your opponent had already gained the 3000 Life Points, then your opponent does not lose the 5000 Life Points. If "Blade Knight" attacked "The Immortal of Thunder" while it was face-down, your opponent neither gains nor loses Life Points from "The Immortal of Thunder"’s effect.
If "Blade Knight" destroys "Spear Cretin", "Spear Cretin"’s effect is not applied.
The effect of a Flip Effect monster destroyed by "Blade Knight" is negated while the destroyed card is in the Graveyard. If the monster is Set again on the field, then the Flip Effect is activated when it’s flipped again.
If "Blade Knight" attacks a face-down Attack Position Flip Effect Monster with equal ATK, and both monsters are destroyed, then "Blade Knight"’s effect is applied because it destroyed the monster, and the monster’s effect is negated.