“They used to be blue, but that changed, to dissociate from the Zionist flag. ”
This is definitely not true. Most taleisim had black stripes for centuries before there was a zionist flag (which is in fact based on those taleisim with blue stripes, but they were a minority).
I read somewhere a suggestion that the stripes on a talles come from the Roman toga, which had stripes on it. Thus it would make sense that sefardim, whose ancestors and traditions did not come from the Roman empire, would not have this custom.