On any shard other than Atlantic, I'd say it's not worth it because the castle is worth far less than the blaze cu, and you could just bring the cu to Atlantic to sell and then bring the gold back to your home shard while keeping significant profit.
On Atlantic, the blaze cu is not worth enough to trade for a castle because like Archnight said the cu is like 2 platinum and the castles are like 10ish platinum on average. So that would be an awesome deal for you if someone offered to give you a castle for it. The cu probably is worth enough to trade for a felucca keep on Atlantic though.