This is the first dor beetle of the year. He was making his way across a sheep-mown field in the rain, and making very heavy weather of it.
Dors are big beetles, this one being almost an inch long. Although they look black, in certain lights and from certain angles they have a beautiful purple-blue sheen to their carapace. This was best seen back in July 2014 when some dors were seen falling around - blog entry here.