The time dependence of dynamical dark energy naturally gives rise to cosmological 'hair' around a black hole. I will introduce a recently identified stable such solution for the cubic Galileon, a concrete and illustrative scalar dark energy example and the currently only known stable solution of this kind. The nature of the scalar hair around these local black hole solutions encodes cosmological information, enabling a new probe of cosmological dark energy dynamics with black hole observations. Specifically, I will show how the quasi-normal mode (QNM) spectrum characterising black hole ringdown can be affected and discuss observational prospects for detecting this. In addition, I will also discuss the broader space of dark energy tests we are currently pursuing within the LVK collaboration - especially modified dispersion relation and parametrised gravitational wave propagation tests - and highlight when and where these can provide additional novel insights into the nature of dark energy.