  We present initial results from a wide-field, multi-colour imaging project,
designed to study galaxy evolution in X-ray selected clusters at intermediate
(z~0.25) and high redshifts (z~0.5). In this paper we give blue galaxy
fractions from eight X-ray selected clusters, drawn from a combined sample of
three X-ray surveys. We find that all the clusters exhibit excess blue galaxy
populations over the numbers observed in local systems, though a large scatter
is present in the results. We find no significant correlation of blue fraction
with redshift at z>0.2 although the large scatter could mask a positive trend.
We also find no systematic trend of blue fraction with X-ray luminosity. We
show that the blue fraction is a function of (a) radius within a cluster, (b)
absolute magnitude and (c) the passbands used to measure the colour. We find
that our blue fractions (f_b) from galaxy colours close to restframe (U-B),
f_b~0.4, are systematically higher than those from restframe (B-V) colours,
f_b~0.2. We conclude this effect is real, may offer a partial explanation of
the widely differing levels of blue fraction found in previous studies and may
have implications for biases in optical samples selected in different bands.
  While the increasing blue fraction with radius can be interpreted as evidence
of cluster infall of field galaxies, the exact physical processes which these
galaxies undergo is unclear. We estimate that, in the cores of the more massive
clusters, galaxies should be experiencing ram--pressure stripping of galactic
gas by the intra--cluster medium. The fact that our low X-ray luminosity
systems show a similar blue fraction as the high luminosity systems, as well as
a significant blue fraction gradient with radius, implies other physical
effects are also important.
