Probably it was an extra fill applied at the text object level.
Text objects can have color applied at the whole object level, and at the character level. (Similarly, groups can have color applied at the group level and at the level of the paths within the group.) The levels of color are stacked like layers, and the front-to-back ordering of those layers is controlled by the Appearance palette.
If you still have your original unfixed file, select the text and open the Appearance palette. See if you have an additional fill layer applied at the object level. From your description, it sounds like it was below the Characters level.
In Illustrator 9, it was easy to accidentally get color applied at the text object level if you used the eyedropper tool to apply color to a selected text object. (The other methods of applying color such as the Swatches palette and the Color palette required you to go to the Appearance palette and explicitly select Add New Fill or Add New Stroke.) The eyedropper tool has been brought into line with the other color application methods, so it shouldn't happen accidently anymore. I forget whether this change happened in AI 10 or AI CS.