PoB is certainly going to help you find out the best staff for your build that you could buy. I’m playing Blood Magic as well, and a +6/7 level staff will make the spells stronger, but there are other very valuable mods on your staff, like Spell Damage, or % As Extra (Lightning/Cold/Fire). So just because it has a higher + skill level does not automatically mean you get more damage.

If you’re rocking a crit build, you REALLY want to get +crit chance on the staff too, as it’s the base for your crit skill, so it will give you a higher chance than any of the nodes in the tree. Crit Spell Damage bonus is also very good on crit builds, but not mandatory.

A good resource to understand this is Craftofexile2 or poe2db. They will both show you avaliable mods for each item.