You means the culling strike support gem ? It only cull rare and uniques mobs, meaning yellow mobs and bosses, not normal mobs. Maybe you tried against the wront type of enemy.

About critical strike chance.

For attacks, the crit chance is the weapon's critical chance. For spells, if the spell has a crit chance listed, it will use that, if there is not, that means the spell can't crit.

maim band blind work on herald of thunder because the skill hits mobs when it triggers. that's why it works. Some skills have several components, and support gems with work on part of the skill.
Like buffing support gems like vitality and clarity will work on the buff part, and maim and blind on the attack part.

Most of the time you can refer to tags. Note that supports can add additional tags. Like curses don't hit by default, but if you put supports that deal damage when you curse, then the curse will then count as a damaging skill.

thanks for the tip with the curse / damge support which leads to hit support

yes I tried it on a rare, no idea. At least I found a good combination of support skills for siphoning strike.