  Between 2004 and 2009 a sample of 28 X-ray selected high- and
intermediate-frequency peaked blazars with a X-ray flux larger than 2 uJy at 1
keV in the redshift range from 0.018 to 0.361 was observed with the MAGIC
telescope at energies above 100 GeV. Seven among them were detected and the
results of these observations are discussed elsewhere. Here we concentrate on
the remaining 21 blazars which were not detected during this observation
campaign and present the 3 sigma (99.7 %) confidence upper limits on their
flux. The individual flux upper limits lie between 1.6 % and 13.6 % of the
integral flux from the Crab Nebula. Applying a stacking method to the sample of
non-detections with a total of 394.1 hours exposure time, we find evidence for
an excess with a cumulative significance of 4.9 standard deviations. It is not
dominated by individual objects or flares, but increases linearly with the
observation time as for a constant source with an integral flux level of ~1.5 %
of that observed from the Crab Nebula above 150 GeV.
