Does the spell lead blades work on melee weapons that are being thrown? Does the damage increase still function from the throw? So for throwing axes, which are both a melee weapon and one that can also be thrown, would lead blades work?
Yes, the damage increase is conditional on the weapon, not the attack. So since it’s a melee weapon, it gets the bonus—even if it isn’t being used for a melee attack. Considering the flavor of the spell (increasing the weapon’s weight and density), this also makes sense narratively (whereas such ranged attacks with “melee” weapons sometimes don’t, whatever the rules say).
Really, personally, I’d extend this to purely-thrown weapons, too, rather than just those that can do melee or throw. It seems to me that the sorts of thing that shouldn’t benefit are projectile weapons, since a denser bow doesn’t automatically do more damage. The same logic might apply to ammunition, but I’m not sure about that and probably wouldn’t assume.
My counterpoint is "Maybe." It only affects melee weapons. Not thrown weapons. Anything used for ranged attacks becomes a ranged weapon. Which means that as soon as the weapon leaves your hand, it stops affecting the weapon... this is similar to size bonuses from enlarge person stopping on thrown weapons as well, as well as making Gravity Bow it's own spell.
"Melee and Ranged Weapons Melee weapons are used for making melee attacks, though some of them can be thrown as well. Ranged weapons are thrown weapons or projectile weapons that are not effective in melee."
Melee weapons = melee weapon (attack). Throwing them = Ranged weapon (attack). Solid MAYBE from me. I don't know if there is a FAQ on it, but I would call it entirely up to GM interpretation. RAW it's ambiguous at best. RAI it was intended, clearly, to affect melee attacks by melee weapons, and I'd say not ranged thrown attacks. You can literally throw ANY weapon and it becomes a thrown weapon, up to and including any ranged weapon (as an improvised thrown weapon). Should Lead Blades apply to thrown improvised weapons too? Just because you can hit someone with a metal rock? Clearly the intent was "no."