Agree with Chris, please note:
-darkish back with many black feathers.
-extensive dark grey flanks.
-the rump is jet-black. The "id-relevant" upper area is partely hidden, but its extremly rare for a White Wagtail to have the lower and middle rump area as black as this (uppertail coverts are regular nearly blackish in White Wagtail).
-extensive white greater coverts, the visible ones appear all white.
-it seems to have all white greater coverts, White normally has narrower white fringing and darker centers to greater coverts.