  Streamer blowout (SBO) coronal mass ejections (CMEs) represent a particular
class of CMEs that are characterized by a gradual swelling of the overlying
streamer and a slow CME containing a flux-rope structure. SBO CMEs arising from
the streamer flank fall into a special category of SBO CMEs involving three
lower arches under the higher streamer arcade. However, the initiation
mechanism for this special category of SBO CMEs remains elusive, due to the
observational limitations. Here we report critical observations of a SBO CME
associated with the eruption of a polar crown filament that originated from the
streamer flank. The filament slowly rose toward the solar equator with the
writhing motion, and underwent a sudden acceleration before its eruption.
Interestingly, during the rising, the filament fields experienced gradual
external reconnections, which is evidenced by the dip-shaped bottom of the
enveloping flux-rope structure changing from a smooth concave, the slow inflows
($\sim$1.8 km s$^{-1}$) from both the filament fields and the coronal loops
beneath, and the persistent brightenings around the interface between the
filament fields and the coronal loops beneath. The newly formed lower loops at
the filament source and the Y-shaped structure in the stretched tail fields
indicate the internal reconnections for the filament eruption. The clear
signatures of the external and internal reconnections shed light on the
initiation mechanisms of SBO CMEs.
