No Black Holes from Light

tl;dr : enough incoming light will be scattered out of the central region around the focal point that you can't turn a "vacuum" central region into a spherically symmetric non-rotating black hole with masses less than tens of thousands of suns, when you start with a spherically symmetric shell of light.

Choice quote, after equation (9): "The intensity required to form a laboratory-scale kugelblitz (R ~< 1m) would be of approximately 10^83 W/m^2, more than 50 orders of magnitude above the state-of-the-art laser pulse intensities, which reach 10^27 W/m^2. For astrophysical sources, the intensity required is still many orders of magnitude above he highest-intensity sources in the universe, including quasars and supernovae [...] total power input must [be much more than] the bolometric luminosity of the brightest quasars [...] for any kugelblitz radius above the Planck length."