Is the transaction confirmed yet? If it is, there's no way to cancel the transaction. Start by checking your browser history (assuming it's stored) and find which bitblender site you used. If you used bitblender.io (or their onion website), your coins should be safe as they are a reputable service. However, if it wasn't that site, it was likely an imposter and your funds are likely lost forever. Also, if you post the address you sent funds to, it may be possible to find a link, however it is unlikely given that it is a mixer site that would try to prevent tracking.