Well, the blue dolar is not legal, so Xoom can't function as a legal business using it on a daily basis. I believe they are bringing (and taking out) money using the stock market which is legal (CCL and Reverse CCL). The rate today is around ~9,50. Now, Xoom is offering 8,35.. but you need to take into account that if they are indeed using this method, they need to pay for commisions for the purchase/selling of stocks in the US broker and the Arg Broker. That has a cost.. also, the prices are varying everyday, so they may be putting a few cents here to avoid a variation that will make them lose money. Perhaps they are also paying people to handle this which may be impacting in the cost you are going to get in the end.
At least I believe this is the reason.
that makes a lot of sense... I wondered how xoom was able to fund their operations