I have soldered quite a bit with both soldering iron and torch. Sounds like one of two things (or both) is happening. First, if the wire has a coating on it, you will not get the solder to work. The coating must be off. If the wire has the coating and you hit it with the torch, it will turn black right away. Second, you could have applied too much heat before trying to solder, or you aimed the flame around the whole mandrel, turning everyting black from the heat. There is a point at which you apply too much heat and must re sand a joint for a good connection.
If you have the blank in a mandrel, a decent torch should apply more than enough heat, even if the heat is wicking away down the mandrel. If you glowed the copper red, you went to far.
I think if you can soak the wire in some thinner and dry before you wrap it, you will most likely get it done just fine.