  The search for Blank Field Sources (BFS), i.e. X-ray sources without optical
counterparts, paves the way to the identification of unusual objects in the
X-ray sky. Here we present four BFS detected in the Brera Multiscale Wavelet
catalog of ROSAT HRI observations. This sample has been selected on the basis
of source brightness, distance from possible counterparts at other wavelengths,
point-like shape and good estimate of the X-ray flux (f_X). The observed f_X
and the limiting magnitude of the optical catalogs fix a lower limit for our
BFS on f_X/f_opt~40. This value puts them well beyond 90% threshold for usual
source classes once HRI energy band and proper spectral shape are taken into
account, leaving room for speculation on their nature. Three BFS show also
evidence of a transient behaviour.
