Hey guys. I've added blood gems to my Blade of Mercy to deal Slow and Fast poison. But they don't seem to actually work on enemies, when I attack. See screenshot.

My question is then; how do I actually use them and make them work on enemies?

These blood gems are currenlty active on my Blade of Mercy+6. As you can see under "Special Attack" Slow poison has a ratio of 2, and Fast poison has 10.

I got fast poison up to 15.8 but it's really hard to tell how much is enough to poison something before you actually just kill it

You probably need to stack up multiple gems to get it to work effectively, also hit the enemy a lot and probably kill them in the process. Some enemies have around 8k-10k life in NG+ (or NG++) though apparently so maybe it's useful there; multi hit attacks (such as Logarius' wheel 2 Hand dash attack) may proc the poison multiple times.

pretty much only useful for enemies with a lot of hp, you can add poison knives to the mix too that might help.