Beat him first try, all you need to do is stack magic defense: Spell Quartz Ring +3, Dispelling Ring +1, and Black set had me at 612 magic resist. I equipped the Rebels greathshield not even upgraded but didnt need it too much, only thing to note is that he delays his attacks longer than the regular smelter, so hold your dodges a split second longer. He got melted by the roaring halberd w/ dark weapon.

boss messed me up

It is absolutely, 100%, undeniably, irrefutably NOT weak to poison

The hitbox on that damn vertical slam cost me like 3 runs

After burning through so many effigies, I got totally lucky: after killing the second NPC, and with about 50% of his life remaining, Blue just stood there and took my beating. I can't believe how long it took to kill him, even without him moving. Do I feel a little bad cheesing him like this? Maybe if it was the first or second time trying to beat him, but not on the tenth.

Respawns: YES

xbox 360/sotfs version- After defeat he dropped soul of red smelter demon.

This boss is NOT weak to Dark - my Dark Arrows with +10 hunter's blackbow did no significant damage against him. He indeed takes poison significant damage, but takes 7+ arrows.
The Blue Version visual of this boss is quite beautiful, but unfortunately it's Dark Souls 2, hotboxes are broken and the melee combat may be complicated. I gave up after he killed me with 2 attacks and just cheesed him using the npc summons and poison/dark arrows. (I do not recommend Dark arrows, but I only had poison and dark ones).

Maybe these games turned me into a masochist, but the runup really isn't that bad - takes like 30-40 seconds and you usually shouldn't get hit by anything - the boss itself is pretty fun and feels good to dodge, just stay away when he kneels after a jump and keep a life gem active for last phase, other than that, pretty doable in 1-5 tries solo

def not weak to poison.