  We study the properties of Brightest Cluster Galaxies (BCGs) drawn from a
catalogue of more than 69000 clusters in the SDSS DR6 based on the adaptive
matched filter technique (AMF, Szabo et al., 2010). Our sample consists of more
than 14300 galaxies in the redshift range 0.1-0.3. We test the catalog by
showing that it includes well-known BCGs which lie in the SDSS footprint. We
characterize the BCGs in terms of r-band luminosities and optical colours as
well as their trends with redshift. In particular, we define and study the
fraction of blue BCGs, namely those that are likely to be missed by either
colour-based cluster surveys and catalogues. Richer clusters tend to have
brighter BCGs, however less dominant than in poorer systems. 4-9% of our BCGs
are at least 0.3 mag bluer in the g-r colour than the red-sequence at their
given redshift. Such a fraction decreases to 1-6% for clusters above a richness
of 50, where 3% of the BCGs are 0.5 mag below the red-sequence. A preliminary
morphological study suggests that the increase in the blue fraction at lower
richnesses may have a non-negligible contribution from spiral galaxies. We show
that a colour selection based on the g-r red-sequence or on a cut at colour u-r
>2.2 can lead to missing the majority of such blue BCGs. We also extend the
colour analysis to the UV range by cross-matching our catalogue with publicly
available data from Galex GR4 and GR5. We show a clear correlation between
offset from the optical red-sequence and the amount of UV-excess. Finally, we
cross-matched our catalogue with the ACCEPT cluster sample (Cavagnolo et al.,
2009), and find that blue BCGs tend to be in clusters with low entropy and
short cooling times. That is, the blue light is presumably due to recent star
formation associated to gas feeding by cooling flows. (abridged)
