  Optical observations of the Type Ia supernova (SN Ia) 2005bl in NGC 4070,
obtained from -6 to +66 d with respect to the B-band maximum, are presented.
The photometric evolution is characterised by rapidly-declining light curves
and red colours at peak and soon thereafter. With M_B,max = -17.24 the SN is an
underluminous SN Ia, similar to the peculiar SNe 1991bg and 1999by. This
similarity also holds for the spectroscopic appearance, the only remarkable
difference being the likely presence of carbon in pre-maximum spectra of SN
2005bl. A comparison study among underluminous SNe Ia is performed, based on a
number of spectrophotometric parameters. Previously reported correlations of
the light-curve decline rate with peak luminosity and R(Si) are confirmed, and
a large range of post-maximum Si II lambda6355 velocity gradients is
encountered. 1D synthetic spectra for SN 2005bl are presented, which confirm
the presence of carbon and suggest an overall low burning efficiency with a
significant amount of leftover unburned material. Also, the Fe content in
pre-maximum spectra is very low, which may point to a low metallicity of the
precursor. Implications for possible progenitor scenarios of underluminous SNe
Ia are briefly discussed.
