  The surface brightness fluctuation (SBF) method is a powerful tool to derive
distances to galaxies for which single stars cannot be resolved. Up to now, the
method has been calibrated mainly at red colours due to the intrinsic faintness
of blue early-type galaxies. In this paper we address the I-band calibration of
the SBF method at blue colours, the regime of dwarf elliptical galaxies (dEs).
We present deep and wide-field VI photometry of the central Fornax cluster
obtained at Las Campanas Observatory. With these data we perform an SBF
analysis of 25 dEs in the range -16.5<M_V<-11.2 mag, 0.8<(V-I)_0<1.10 mag. For
the calibration analysis we exclude eight dEs whose SBF measurement was
affected by poor seeing (FWHM ~ 1"). Our SBF data are inconsistent at the 3
sigma level with a colour independent absolute SBF magnitude Mbar_I, presenting
a problem for SBF models that predict such a flat relation. There is a weak
indication in our data (1.8 sigma) to favour a two-branch calibration over a
one-branch calibration with broad scatter in the Mbar_I - (V-I) plane. We
obtain the following one-branch empirical SBF calibration:
  Mbar_I=-2.13 (+/- 0.17) + 2.44 (+/- 1.94) * [(V-I)_0 - 1.00] mag.
  We deduce a 0.34 +/- 0.14 mag cosmic scatter of Mbar_I, which is
significantly larger than found at redder colours. This is in agreement with
those theoretical SBF models that predict Mbar_I to be more sensitive to
age-metallicity variations in the blue than in the red. We find evidence that
the fainter galaxies in our samples contain younger and more metal-rich stellar
populations than the brighter ones. The application of our empirical
calibration to published SBF measurements of Hydra and Centaurus cluster dEs
leaves the distances to both clusters unchanged.
