Have just started close-ups with my new macro lens. Most shots turned out great (newbie's standard!), but I'm puzzled about this one in particular:

Why does the yellow edges of the flower have a dark blue outline? Is this natural, something to do with the lens, or my technique?

This was taken with a Sigma 105mm macro lens.

Any comments would be most appreciated, thks!

Note: The smaller images here have a lot of blurry blue lines (due to sizing?), please click on photo to expand to original size where the blue edges remain.