Thanks Akchas!

Out of curiosity, what is this background made from? Is it painted canvas, painted muslin, bleached muslin without paint, seamless paper, painted wall or what?

If it's a bleached muslin, it could be that it's been treated with a chemical whitener, which - when exposed to UV rays - tends to reflect a bluish tint which will definitely show up in your photos. If it's washable, I'd definitely wash it on the gentle cycle and use a Woolite type of detergent. That will reduce or eliminate the whitener and you should get a much more neutral image.

(yes, I see I'm very late to this thread, but it may help the next person )

Thanks. It's white seamless paper--not super white which I know can cast a blue tint--more like "snow". I keep my WB set for outdoor or strobe--this is one area that worked beautifully for when I was shooting transparencies but is a minor problem with digital.