Yeaaaaaaah. One can never have enough unfinished projects lying around.
With regards to black fabric. One might ask, would acquiring black fabrics during the coarse of doing business with Spanish vessels and residing in a region where Spain had a strong influence be much of an issue? Consider Spain's propensity for conservative fashion, the need for mourning dress, the presence of darkly robed Catholic emissaries and their commute over the seas for decades. While I don't recall reading/seeing anything about the disposition of clothing being used for anything other than garments during the period, would this lend itself as a possible source for material with which to stitch black flags?