Bleach-imaged plasmon propagation, BlIPP, is a far-field microscopy technique developed to characterize the propagation length of surface plasmon polaritons in metallic waveguides. To correctly extract the propagation length from the measured photobleach intensity, it is necessary to understand the mechanism by which dye photobleaching occurs. In particular, 1- vs 2-photon bleaching reactions yield different propagation lengths based on a kinetic model for BlIPP. Because a number of studies have reported on the importance of 2-photon processes for dye photobleaching, we investigate here the nature of the photobleaching step in BlIPP. We are able to demonstrate that only 1-photon absorption is relevant for typical BlIPP conditions as tested here for a thin film of indocyanine green fluorescent dye molecules coated over gold nanowires and excited at a wavelength of 785 rim. These results are obtained by directly measuring the excitation intensity dependence of the photobleaching rate constant of the dye in the presence of the metallic waveguide.