The blazar content in the

Aims. We present the results from a study of the blazar content in the 15–150 keV map of the entire sky obtained analysing 39 months of data collected by the BAT telescope aboard the Swift satellite. Methods. We performed a cross-correlation of the significance map, obtained with a dedicated highly efficient algorithm for data processing and image reconstruction of the BAT survey data, with the blazar population of the Roma-BZCAT. After corrections for source confusion and spurious detections, we found significance excesses higher than two standard deviations for 304 sources, and the corresponding fraction of expected spurious associations is ~20%. We selected hard X-ray blazars according to their significance level and carried out a statistical analysis to characterise their emission properties. Results. A sample of 121 blazars detected at a significance level σ > 3 is discussed in greater detail. The fraction of blazars with uncertain classification in this sample is considerable (~23%), more than twice the percentage obtained considering all the blazars classified in the Roma-BZCAT (~9%). We attribute the X-ray flux of the majority of selected BL Lac objects to the synchrotron emission