  Cosmological constraints from galaxy clusters rely on accurate measurements
of the mass and internal structure of clusters. An important source of
systematic uncertainty in cluster mass and structure measurements is the secure
selection of background galaxies that are gravitationally lensed by clusters.
This issue has been shown to be particular severe for faint blue galaxies. We
therefore explore the selection of faint blue background galaxies, by reference
to photometric redshift catalogs derived from the COSMOS survey and our own
observations of massive galaxy clusters at z~0.2. We show that methods relying
on photometric redshifts of galaxies in/behind clusters based on observations
through five filters, and on deep 30-band COSMOS photometric redshifts are both
inadequate to identify safely faint blue background galaxies. This is due to
the small number of filters used by the former, and absence of massive galaxy
clusters at redshifts of interest in the latter. We therefore develop a
pragmatic method to combine both sets of photometric redshifts to select a
population of blue galaxies based purely on photometric analysis. This sample
yields stacked weak-lensing results consistent with our previously published
results based on red galaxies. We also show that the stacked clustercentric
number density profile of these faint blue galaxies is consistent with
expectations from consideration of the lens magnification signal of the
clusters. Indeed, the observed number density of blue background galaxies
changes by ~10-30 per cent across the radial range over which other surveys
assume it to be flat.
