On 17/06/2012 3:44 a.m., RichA wrote:
Heat expansion and contraction, which effects optical performance. So
they've introduced something else to compensate for it. Because
aluminum and magnesium don't suffer nearly as much thermally-related
shape changes as plastic, older lenses and new high-end principally
metal lenses do not have the problem.

http://www.dpreview.com/news/2012/06...superzoom-lens


Thom Hogan sums it up nicely in his website:

"Of course, if they didn't use black they might not absorb so much heat
in the first place"