From a 1962 banner ad. just prior to Supergirl coming out of her Superman-imposed secrecy.
So the colorist thought she had... black hair? Well, it would have made sense since she shares some DNA strands with Superman and HE'S got black hair. Except the Maid of Steel had been around since 1959, so really there's no excuse. Here's the actual cover as it was eventually published:
It's a well-known fact the blond gene jumps around in dark-haired families. Lois and Lucy Lane. Superman and Supergirl. Pete Ross from the comics and Pete Ross from the Smallville TV show. WELL. KNOWN. FACT.