Application of the J04 criterion: observed upper limits in the R_C band relative to the measured flux density at 1.73 keV (the logarithmic mean of the Swift/XRT window, 0.3 − 10 keV) for the 17 bursts in our sample. When no R_C-band data were available, we used the observed spectral slope β_OX to shift the flux density from the native filter to the R band (Table 2). The bursts falling in the gray area fulfil the J04 criterion. The three bursts that can be assumed to be securely classified dark bursts according to J04 as well as V09 (see Fig. 3) are marked with a filled black triangle (see Sect. 2.3).