I would be a bit concerned, actually, and send the client on to see the doctor. If the doctor found nothing, then a chiropractor might locate vertebral misalignment. If both those check out fine, then I'd be comfortable working the scalenes, etc. again. It could be a silent migraine that causes the blurred vision.
How do you work the scalenes? Possibly just backing off, working lighter and being willing to take several sessions to get to the deeper levels would alleviate some of the symptoms. Gentle stretching techniques could help soften the muscles so that you can make a bit more progress with each massage session. Too much too fast can cause rebound headaches and many other symptoms.