What should I do when it rains blood?

The Answer is tricky but simple. When it rains blood, there is a shield that forms around the boss(Dominus). If you are outside of this circle you will receive increasing stacks of the corrupting blood debuff, which causes damage over time. Just enter this area and you will be protected from getting any more stacks of corrupting blood.

To remove any stacks of corrupting blood from before you got inside the circle use a flask with the "of Staunching" suffix, or just wait for them to dissipate on their own.

You will be in range for his close range melee attack so be careful and kite him in that small area if possible or just heal up.

Here are 2 guides by ZiggyD that explain the Dominus fight mechanics in detail

I know this is old but i just came across this boss and had the same question (hence why im here).

I actually beat him in hardcore just now using a different method.

I would create a town portal. Once it starts raining jump in the portal and wait for 20-seconds then go back in, create another portal (in-case it rains again). Rinse n repeat

Obvious down side is you are using town portals and it feels a lil like cheating. Up side is u dont have to toe 2 toe this beast.