I think it is badly mixed metal. Silver in purified form has to be alloyed to reduce it to the correct fineness (92.5% is sterling silver). The normal metal added would be copper.
I have noticed a far greater tendency for silver to suffer from haymarking than gold. I think this is due to the difference in melting points between the three metals. Silver melts at 100 degrees below the melting point of copper, so the pot needs to be heated far beyond the point that liquid silver forms in order to melt any added copper. Failure to melt the copper would result in specks of sold copper remaining as it is likely the lighter particles would be in suspension. Gold on the other hand melts within 20 degrees of copper and so there is less chance of the copper failing to melt. Time allowed for melting is another factor. Once poured and cooled, any copper will react with the air to make copper (II) oxide, which is black.