This is fixed in 0.9.14 upstream, by renaming bsetroot to fbsetroot. From the changelog:
*05/07/12: Renaming of 'bsetroot' to 'fbsetroot' (Mathias) blackbox's 'bsetroot' and fluxbox's 'bsetroot' have different syntax nowadays and collide when you install fluxbox and blackbox. hence the renaming updated manpage, helper utils (fbgm, startfluxbox, fbsetbg), nls etc -- I usually have a GPG digital signature included as an attachment. See http://www.gnupg.org/ for info about these digital signatures.
signature.asc
Description: Digital signature