[4.6]–[12]/[3.4]–[4.6] MIR colour–colour plot reporting the positions of gamma-ray emitting blazars (in cyan) associated with WISE sources forming the blazar strip (see Massaro et al. 2013a for more details), together with the BL Lac objects identified in this paper. Filled and unfilled squares depict the proposed associations flagged with L or P in the last column of Table 3, respectively.