The most value you can get out of a double slotted blank weapon is as a min.lvl4 leveling machine.
You want keen on there and good(2d6).
In addition you want to put in 2 damage rubies as well.
That gets you a keen+6d6 weapon that will last you perfectly fine till level.12 where you can get improved critical.
After that you can take away the keen and add another 4d6 piercing dmg and vampirism.
