Blue Mystery

Walking down near the river Ouse this morning I came across an area of ground that was covered in the sawn off stems of a large umbellifer species. There were probably more than a hundred of these stems ranging from 20 to 30 cm in height and perhaps a third of them were bright blue in colour. I'm curious as to what this is and what has caused the blue colouration. My current theory is that the plant was giant hogweed (although I've walked this way many times and don't recall seeing it) and the colouration is from some chemical control. But, if that's the case, then why aren't all of the stems discoloured? and, would you use a chemical control and cut the plant down? Alternative suggestions welcome.....