  We have observed three clusters at z~0.7, of richness comparable to the low
redshift sample of Butcher & Oemler (BO), and determined their fraction of blue
galaxies. When adopting the standard error definition, two clusters have a low
blue fraction for their redshifts, whereas the fraction of the third one is
compatible with the expected value. A detailed analysis of previous BO-like
studies that adopted different definitions of the blue fraction shows that the
modified definitions are affected by contaminating signals: colour segregation
in clusters affects blue fractions derived in fixed metric apertures,
differential evolution of early and late type spirals potentially affects blue
fractions derived with a non standard choice of the colour cut, the younger age
of the Universe at high redshift affects blue fractions computed with a colour
cut taken relatively to a fixed non evolving colour. Adopting these definitions
we find largely varying blue fractions. This thorough analysis of the drawbacks
of the different possible definitions of the blue fraction should allow future
studies to perform measures in the same scale. Finally, if one adopts a more
refined error analysis to deal with BO and our data, a constant blue fraction
with redshift cannot be excluded, showing that the BO effect is still far from
being detected beyond doubt.
