A black hole shadow is the dark region of the sky centered on a black hole when viewed from afar. The black hole blocks light coming from behind, so, for example, a star situated in a line with the black hole behind it won't be seen. This shadow can be highly obscured by electromagnetic radiation (EMR) generated around the black hole if accretion is taking place, resulting in thermal emission from heating under pressure and various forms of non-thermal emission from gas's interaction with electric fields and magnetic fields. Viewing the shadow requires viewing at a wavelength such that there is a high contrast between EMR from the shadow's foreground, and EMR just outside the edge of the shadow (as well as being a wavelength that penetrates clouds further in front of the black hole). For current efforts to view supermassive black holes, wavelengths on the order of a millimeter are used (millimeter astronomy, at the border between microwave and infrared).
Also, the shadow's size is affected by the significant bending of light paths by the strong-field gravity. The size of the shadow is determined by the trajectory of photons, the shadow disk's edge being at the closest points in the sky that photons end up aimed at the observer. The size of a black hole is "by definition", often thought of as the size of the event horizon. The visible dark disk has a larger diameter than the event horizon, for a Schwarzschild black hole, being 271/2/2 larger. A bright ring (photon ring) around the shadow is light redirected from near and distant sources, some of which has encircled the black hole more than once, which can be discerned through the fine detail of the ring, which consists of multiple concentric rings. The ring's appearance (and perhaps the shape and size of the shadow) is also affected by the black hole's rotation as well as transients in the effects of any accretion, and the ring is generally asymmetric, brightest where the rotation is toward us, and constantly changing.