We examined the complete set of observation of GRB X-ray afterglows performed by BeppoSAX up to February 2001. We do not find increased X-ray absorption in GRBs without optical transient (dark GRBs) compared to GRB with optical transient (OTGRBs). Rather, we find that X-ray flux of dark GRB is similar or equal to 5 times lower than that of OTGRBs. This result, which is significant at 3sigma level, could suggest that the dark GRBs are uncaught in the optical band just because they are faint sources. To test this hypothesis, we calculated the optical-to-X flux ratios of the GRBs of our sample. We find that, while 75% of dark GRBs have flux ratio upper limits still consistent with those of OTGRBs, the remaining 25% are similar or equal to 5 times weaker in the optical than in X-rays. We discuss the possible causes of this behaviour, including a possible occurence in high-density clouds or origin at very high redshift.