On embargo laws, the notion that another country can use Cuban tobacco in the non Cuban country to import into the US is not accurate. Were this the case, Cuban tobacco cigar rolled in the Dominican Republic would be all over the US market compliments of Altadis.
Under US import laws, the US Customs and Border Patrol requires that the importer disclose the country of origin. Cuban tobacco in a cigar rolled in the Dominican Republic with Dominican Republic tobacco would still require disclosure of the Cuban tobacco and hence application of the embargo.
If a cigar imported into the United States uses tobacco from multiple countries, all the countries, legally, have to be disclosed. So this cigar would not be legally importable as the disclosure on the Cuban tobacco would result in application of the embargo, and since the embargo cannot be applied just to the Cuban tobacco, it means no cigar.
If the importer can show the tobacco was acquired prior to enactment of the embargo, they can use it and import it.
Now, do all importers follow US Customs law ? Probably not.
I think one reason no blend have occurred prior is because it did not make sense economically. But if the price Padron 50th is an economic indicator on Nicaraguan tobacco, maybe these economics have changed.