Hallo, Sue,
wie übersetzt man das richtig:
"He (gemeint ist eine Person) is a dark horse."
... dass es "unbeschriebenes
Blatt" oder "unbekannte Größe (methaporisch)" heissen soll.
das dürfte auch die treffende Übersetzung sein.
dark horse
n.
-One who achieves unexpected support and success as a political candidate, typically during a party's convention.
-A little-known, unexpectedly successful entrant, as in a horserace.
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A little known, unexpectedly successful entrant, as in You never can tell some dark horse may come along and win a Senate seat. This metaphoric expression originally alluded to an unknown horse winning a race and was so used in a novel by Benjamin Disraeli (The Young Duke, 1831). It soon began to be transferred to political candidates, among the first of whom was James K. Polk. He won the 1844 Democratic Presidential nomination on the eighth ballot and went on to win the election
(The American Heritage® Dictionary of the English Language, Fourth Edition)
Gruß
Kreszenz