Blonde parents dark haired baby

So DH and I are both blonde we have a 5 year old who like us is blonde, I just gave birth to our second little girl and she came out with a full head of dark hair.?.?. I know that hair color can be a result of so many different genes but this was definitely unexpected and the "so who's the dad" remarks and the "oh must be the milk mans" comments are starting to get kind of annoying. 
Anyone else's ever experience this? 
Dh and I are both the only blondes in our families...