Fully!
Judge's Mien specifically says a "100% chance" for this reason, rather than stating it definitely, as you see for example with Chill effects. If the target is Light resistant, it will reduce the chance. Also, your Cunning and the target's Vitality still play a role too: even a non-resistant target may resist if their Vitality bonus > Cunning bonus.
As for Reaper's Mien: it gets more interesting!
![Very Happy :D](./images/smilies/icon_e_biggrin.gif)
Drain effects can come in two kinds:
1) Damage type and drain type match. In this case, if the target resists, the drain amount is only penalized once: when the damage itself is. So if you use Life Siphon (a Body damage spell and Body drain type) on a Body resistance target and say, the damage would have originally been 10, it gets knocked to 3.33, which is reduced 50% for the drain rate, leaving you with 1.66.
2) Damage type and drain type don't match. In this case, both resistances can reduce the final drain result. Leech Bite is a good example of this type: Pierce damage, but Body drain type. If the target resists both Pierce and Body, your 10 damage would be reduced 5... then the 50% drain would be cut to 16%... and the final result would be less than 1.
If Reaper's Mien triggers off a Dark attack, it functions as Type 1, since itself is Dark.
If Reaper's Mien triggers off a Light attack, it functions as Type 2, since it's Dark type is a mismatch with Light.
All that said.. in the vast majority of cases, the difference is pretty minor. It only really gets noticeable if the target resists both types: it's basically impossible to drain anything with most abilities in that case.