The other day I made a question about the engine painting and why it isn't painted at the front part. The answer was: to facilitate anti-ice system.
Source: fervegeon.com
So perhaps the leading edge of the horizontal and vertical stabilizer have a sort of anti-ice system?
And this is not only on the 787; the 777 also has the leading edges unpainted.
Source
And on the 757 and winglet.
Source: flickr