English portraits of the 18th century were once among the bluest of blue chips in the art market. In 1921, U.S. Railroad Heir Henry E. Huntington plunked down better than $500,000 for Thomas Gainsborough's Blue Boy, setting the record for English canvases. Hundreds of other rich Americans were supplying themselves with high-priced ancestral portraits from England at about the same time. But the fashion waned and almost disappeared until last week, when Gainsborough's Mr. and Mrs. Robert Andrews fetched a fat $364,000 at auction in London.
The seller: the great-great-great-grandson of the Andrews couple, who posed under a tree that is still...