Two genera of Dragonflies (Anisoptera) have hind-wings with a dark patch at the base: Chasers (Libellula) and White-faced Darters/Whitefaces (Leucorrhinia), of which there is but one species in the UK.
These genera can be distinguished by the number of antenodal cross (Ax) veins.
Less than 10 antenodal cross veins – Whiteface
More than 12 antenodal cross veins – Chaser
OR, if the detail of the Ax veins is not visible, what about the pterostigmas instead?
short pterostigmas, thick front to back – Whiteface
longer, narrower pterostigmas – Chaser