Concorde flew to an altitude of 60,000 ft (18.3 km) where stars should be visible at noon, aren't they? This question asks on how high stars become visible, and it is said Blackbird pilots could see them at 80,000 ft. Concorde flew at the Armstrong line when highest, above more than 90% of the atmosphere's mass. Are there reports of passengers and pilots on Concorde seeing stars (other than the Sun) when at peak altitude? Also, did the Sun appear whiter due to the thinner atmosphere?
Yes, in at least one exceptional circumstance. The book Racing the Moon’s Shadow with Concorde 001 describes a 1973 scientific flight that remained in the moon's umbra for more than an hour, in daytime. As stars are quite visible from the ground during a solar eclipse's totality, they would have been even more visible from a 17 km altitude.
This page has some images of the view from the Concorde. On one image two stars can be seen. So obviously one could see some.