We present the results of a study on the blazar content in the 15-150keV map of the entire sky obtained analyzing 39 months of data collected by the BAT telescope aboard the Swift satellite. We performed a cross-correlation of the significance map, obtained with a dedicated highly efficient algorithm (Segreto et al., 2010A&A...510A..47S) for data processing and image reconstruction of the BAT survey data, with the blazar population of the Roma-BZCAT (Massaro et al., 2009, Cat. J/A+A/495/691). After corrections for source confusion and spurious detections, we found significance excesses higher than two standard deviations for 304 sources; the corresponding fraction of expected spurious associations is about 20%. We selected hard X-ray blazars according to their significance level and carried out a statistical analysis to characterise their emission properties. A sample of 121 blazars detected at a significance level sigma>3 is discussed in greater detail. The fraction of blazars with uncertain classification in this sample is considerable, more than twice with respect to the percentage obtained considering all the blazars classified in the Roma-BZCAT. We attribute the X-ray flux of the majority of selected BL Lac objects to the synchrotron emission.