  Several properties of canonical quantum gravity modify space-time structures,
sometimes to the degree that no effective line elements exist to describe the
geometry. An analysis of solutions, for instance in the context of black holes,
then requires new insights. In this article, standard definitions of horizons
in spherical symmetry are first reformulated canonically, and then evaluated
for solutions of equations and constraints modified by inverse-triad
corrections of loop quantum gravity. When possible, a space-time analysis is
performed which reveals a mass threshold for black holes and small changes to
Hawking radiation. For more general conclusions, canonical perturbation theory
is developed to second order to include back-reaction from matter. The results
shed light on the questions of whether renormalization of Newton's constant or
other modifications of horizon conditions should be taken into account in
computations of black-hole entropy in loop quantum gravity.
