  Context: Type Ibn supernovae are a rare class of stripped envelope supernovae
interacting with a helium-rich CSM. The majority of the SNe Ibn reported
display a surprising homogeneity in their fast lightcurves and starforming
hosts. Aims: We present the discovery and study of SN 2020bqj (ZTF20aalrqbu), a
SN Ibn with a long-duration peak plateau lasting 40 days and hosted by a faint
low-mass galaxy. We aim to explain its peculiar properties using an extensive
data set. Methods: We compare the evolution of SN 2020bqj with SNe Ibn from the
literature. We fit the bolometric and multi-band lightcurves with different
powering mechanism models. Results: The risetime, peak magnitude and spectral
features of SN 2020bqj are consistent with those of most SNe Ibn, but the SN is
a clear outlier based on its bright, long-lasting peak plateau and low host
mass. We show through modeling that the lightcurve can be powered predominantly
by shock heating from the interaction of the SN ejecta and a dense CSM. The
peculiar Type Ibn SN 2011hw is a close analog to SN 2020bqj, suggesting a
similar progenitor and CSM scenario. In this scenario a very massive progenitor
star in the transitional phase between a luminous blue variable and a compact
Wolf-Rayet star undergoes core-collapse, embedded in a dense helium-rich CSM
with an elevated opacity compared to normal SNe Ibn, due to the presence of
residual hydrogen. This scenario is consistent with the observed properties of
SN 2020bqj and the modeling results. Conclusions: SN 2020bqj is a compelling
example of a transitional SN Ibn/IIn based on not only its spectral features,
but also its lightcurve, host galaxy properties and the inferred progenitor
properties. The strong similarity with SN 2011hw suggests this subclass may be
the result of a progenitor in a stellar evolution phase that is distinct from
those of progenitors of regular SNe Ibn.
