Buckskins always carry one cream gene -- it's what dilutes the color to be buckskin instead of being a bay color. But if we believe the registry for the colors of her offspring, she hasn't thrown it in the past. But, having had 2 'black' offspring, it's extremely possible that one or both of them were really smokey-blacks - which would mean they got a cream gene from her. Lots of people look at 'black-looking' offspring and register them as black, when they are really smokey blacks.
Congratulations on your upcoming foal! I hope you'll join us on the Mare/Foal forum here on LB. We'd love to see what new little present she has for you this year!!