Modern Greeks are a lot more slavic and mediterranean than ancient greeks were. Due to lots of barbarian invasions after the collapse of Rome. Besides, the Macedonians were a different racial group entirely, and could have been blonde. And anyway contemporary historians attested to Alexander being blonde.
Only thing I don't agree with in the original post is that in 300 years it is impossible that there wasn't some interbreeding between the Ptolemies and the local Egyptians, so Cleopatra could have been dark afterall.
It is not at all far-fetched that the Ptolemies never interbred with the native Egyptians. The Ptolemies were so concerned with purity of blood that they quite frequently practiced intermarriage within the royal family. Cleopatra's first husband, for instance, was her brother.
The Greeks did not like mating with others. That does not prove anything, but I can imagine them keeping them gene pool Greek. Alexander the Great's men were appalled at his willingness to mix with other races.
Modern greeks are more mediterranean than ancient greeks? Perhaps you'd like to rethink this statement.