I steam my broccoli until it's 'al dente' and some buds turn brown. I've done a small search, but haven't found anything.    I'm thinking maybe the steam is too hot or I'm steaming too long. What do you think? 
further to SAJ14SAJ's answer, what happens is the Magnesium atom in the bright green Chlorophyll is lost in acidic conditions and you end up with Pheophytin which is Olive Green.  There are other reactions at play mediated by the enzyme Chlorophyllase which can be active even in frozen storage.  The main reason for blanching (hot water part) is to stop this enzyme's activity.  The ice bath is to stop the cooking process and the magnesium loss.  This short presentation from Purdue University explains this along with the molecular structure if you're interested.  The presentation states that green vegetables are blanched to end enzyme activity, to preserve the natural green colour. 