Game Wide you have about a 10% drop rate on Cobalt. Most mobs dropping it are in the 68-72 lvl range. The buried sea Sirens seem to have that type of drop rate although other mobs in that zone drop it too.
I recently worked all my trade skills up to 300. While doing smithing I spent some time farming cobalt. I ended up farming in two places using cwtn plugins. I used plane of Fire and cleared the entire area in the south, up to the where the two Phoenix raid mobs spawn. There is a spot on the southern wall of the zone next to the wall that surrounds the area with the fire giants and tables camp where if you set your radius to 1600 -1650, it’ll will clear everything in that field, and all of tables area, but won’t go as far as the fort that splits the zone, nor try to navigate the fire tunnels where it’s a nightmare for getting stuck.
The other zone I camped was the accursed nest. If you zone in and face west, set the radius for 5000 and the pull arc to 180, it’ll have more mobs that it can keep up with and will ignore the keyed part of the zone.
Between these two zones I got plenty of cobalt, and a decent amount of vendor trash, plus various other trade skill items like silks, pelts and gems. The cobalt you’ll get from each of these zones is probably less than 50 per hour, but I think that’s about par the course for these things. I don’t think there’s a magic zone where you can get tonnes of this stuff.
If your levelling smithing, once you’re through the cobalt barrier however, there is a really great zone, and area for farming all the ones you need to reach 300 plus max your trophy. Shards landing. The arena area drops a great range of tantalum, tungsten, vanadium, osmium and scorched ore. All of which can be combined to create studs, all of which have trivials over 300.