I don't know the vendor - which means nothing one way or the other. They claim they offer a 30 days money-back guarantee, but I cannot get details showing on their website; try giving them a call and ask if it has any limitations (e.g. restocking fees or money back only given if the item does not match the description).
Assuming it's a "real" money back guarantee (i.e. 100% of money refunded - minus possibly shipping costs and credit card commissions - for any reason), you have plenty of time to get the stone identified and matched to the report by a good appraiser - which for something costing $1000 or above (and virtually identical to a treated stone costing 1/5 to 1/10 of the price) I would strongly recommend you do, unless you know and trust the vendor.
I am slightly puzzled by the choice of a (relatively) expensive natural black - usually people purchasing black diamonds do so for the appearance, and treated (irradiated) blacks are usually even blacker and more uniform than naturals. Is this a deliberate decision?