  The Arnowitt-Deser-Misner formalism is used to derive variations of mass,
angular momentum and canonical energy for Einstein-Maxwell {\it dark matter}
gravity in which the auxiliary gauge field coupled via kinetic mixing term to
the ordinary Maxwell one, which mimics properties of {\it hidden sector}.
Inspection of the initial data for the manifold with an interior boundary,
having topology of $S^2$, enables us to find the generalised first law of black
hole thermodynamics in the aforementioned theory. It has been revealed that the
stationary black hole solution being subject to the condition of encompassing a
bifurcate Killing horizon with a bifurcation sphere, which is non-rotating,
must be static and has vanishing {\it magnetic} Maxwell and {\it dark matter}
sector fields, on static slices of the spacetime under consideration.
