Well, as Sherlock Holmes famously said, "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth. " but it still doesn't sound like the background to me. I don't claim to be an expert on the subject and I often get things wrong anyway, but white vinyl (and most other materials tend to go yellow over time, not blue, although I suppose there is a faint possibility that the manufacturers have added blue to mitigate the inevitable future yellowing - in which case the problem will of course resolve itself over time.
Personally I always prefer to shoot against a grey background and then cut out in PP, which takes hardly any time and produces much better results, but we all have our own way of doing things.