The equilibrium structures, binding energies, and vibrational spectra of the cyclic, hydrogen-bonded complexes formed between formaldehyde, H2CO, and hydrogen fluoride clusters, (HF)(1 <= n <= 4), are investigated by means of large-scale second-order Moller-Plesset calculations with extended basis sets. All studied complexes exhibit marked blue shifts of the C-H stretching frequencies, exceeding 1.00 cm(-1) for n = 2-4. It is shown that these blue shifts are, however, only to a minor part caused by blue-shifting hydrogen bonding via C-H... F contacts. The major part arises due to the structural relaxation of the H2CO molecule under the formation of a strong C=O center dot center dot center dot H-F hydrogen bond which strengthens as n increases. The close correlation between the different structural parameters in the studied series of complexes is demonstrated, and the consequences for the frequency shifts in the complexes are pointed out, corroborating thus the suggestion of the primary role of the C=O center dot center dot center dot H-F hydrogen bonding for the C-H stretching frequency shifts. This particular behavior, that the appearance of an increasingly stronger blue shift of the C-H stretching frequencies is mainly induced by the formation of a progressively stronger C=O center dot center dot center dot H-F hydrogen bond in the series of H2CO center dot center dot center dot(HF)1 <= n <= 4 complexes and only to a lesser degree by the formation of the so-called blue-shifting C-H... F hydrogen bond, is rationalized with the aid of selected sections of the intramolecular H2CO potential energy surface and by performing a variety of structural optimizations of the H2CO molecule embedded in external, differently oriented dipole electric fields, and also by invoking a simple analytical force-field model.