PDSM35 is reporting control blocks that are created and maintained by IBM LLA code. So, PDSMAN does not set any values - we just report what is there. The bottom line is we have no control over that and cannot answer the question about what the difference is between blank and zero.
As per LLA Extensions and Performance Guide, "Recent Accesses" is described as: The number of times the module has been fetched. If blank, this member has not been fetched since LLA was last activated, maybe, 0 value has the same meaning but, as commented, this is not under our control