I always carry around a black & blue pen (and sometimes red, I like pens*). At some point I got into the habit of always using blue to sign; not clear why. I distinctly remember signing something in blue and then having to do it all over again because they insisted that I only use black ink. I am now suspicious of this this past event..

*not the expensive kind either, I've tried pens that were $50+; fav is still a $2.75 Uniball Jetstream 1.0mm. Smooth pens make pen&paper writing/signing fun