Twitter widget shows black text on black background

I have added a number of widgets to the right sidebar including recent posts, categories and twitter. All have a grey/black title background with white text except for the twitter widget. This is showing as grey/black text which means it looks like it is not displaying the text but if I highlight the area with my mouse I can see it. Not sure if this is different to other sidebar headings as twitter widget heading is showing as a hyperlink.

Issue can be seen here: http://www.hellblokk.com/