http://demonssouls.wikidot.com/blueblood-sword
This is a unique sword. it needs fairly high stats to wield but it is worth it. It is unique in that Luck affects both the Physical and Magical Damage. So if you have a high Luck stat (30+) then this could very well be the best sword in the game for you.
At max stats the Blueblood Sword has 100 + 145 Physical Attack and 100 + 145 Magical Attack.
At min stats (base requirements + 7 Luck) the Blueblood Sword has 100 + 36 physical and 100 + 36 magical attack. Compare to a Dragon Long Sword +4, which has 133 physical and 133 fire attack.
The Blueblood Sword can also be enchanted. Fast swing speed, good range, and magic damage already factored into the enemy defense makes it probably the best option for casting Light Weapon.
et (toujours) en toute humilité