What is this blue insect?
I am pretty sure that this is an oil beetle (also known as blister beetle) of the family Meloidae. In this special case this is probably the violet oil beetle (Meloe violaceus), although this does not completely coincide with the size you mention. The slightly different color is probably caused by the different lighting.

See the image from this Wikipedia for comparision:



Be careful with them, as they can secrete a poisonous substance making blisters on the skin (hence there name) when they feel attacked. For more information see also here.