Hi Ben.
I'm thinking that if you already have a raster, first clean black pixel and after convert to polygon if you need it.
You can use the SetNull function to operate with pixel values, and obtain a new raster keeping only the pixel you want to. Into the sentence, the SetNull function will give the 'NoData' value to the pixel that will match the expression. The rest will remain with the value you want, for example 1.
It will be something like (Assuming that the black pixels have a 0 value):
# Check out the ArcGIS Spatial Analyst extension license
arcpy.CheckOutExtension("Spatial")
# Execute SetNull
outSetNull = SetNull(BlackPixRast, 1, "VALUE == 0")
# Save the output
outSetNull.save("C:/TEMP/newRaster")
# Convert To Polygon
arcpy.RasterToPolygon_conversion(newRaster, BlackPixPoly, "SIMPLIFY")
Good luck!
Luis Pascual