Context :

Aims : We performed observations with the Effelsberg 100-m radio telescope to measure flux densities and polarised emission of sources selected from the “ Deep X-ray Radio Blazar Survey ” ( DXRBS ) to better define their spectral index behaviour in the radio band , with the aim to construct a homogeneous sample of blazars .

Methods : Sources were observed at four different frequencies with the Effelsberg 100-m telescope .
We complemented these measurements with flux density data at 1.4 GHz derived from the NRAO VLA Sky Survey ( NVSS ) .

Results : The spectral indices of a sample of faint blazars were computed making use of almost simultaneous measurements .
Sixty-six percent of the sources can be classified as “ bona fide ” blazars .
Seven objects show a clearly inverted spectral index .
Seventeen sources previously classified as flat spectrum radio quasars ( FSRQ ) are actually steep spectrum radio quasars ( SSRQ ) .
The flux densities obtained with the Effelsberg 100-m telescope at 5 GHz are compared with the flux densities listed in the Green Bank ( GB6 ) survey and in the Parkes-MIT-NRAO ( PMN ) catalogue .
About 43 % of the sources in our sample exhibit flux density variations on temporal scales of 19 or 22 years .

Conclusions : We confirm that 75 out of 103 sources of the DXRBS are indeed FSRQs .
Twenty-seven sources show a spectral index steeper than -0.5 and should be classified as SSRQs .
Polarised emission was detected for 36 sources at 4.85 GHz .
The median value of the percentage of polarised emission is ( 5.8 \pm 0.9 ) % .
Five sources show rotation measure ( RM ) values > 200 rad m ^ { -2 } .