Does Hebrews 9:15 teach that those that God chose as righteous under the old covenant also had to wait for the blood of Christ for the promise to come into effect?
Salvation to mankind required the offering of a perfect sacrifice for mankind's sins. "For it is impossible for the blood of bulls and goats to take away sins" (Hebrews 10:4). Thus, Jesus' sacrifice was key to the forgiveness of everyone's sins, including those who lived before Jesus. "For this reason He is the mediator of a new covenant, so that, since a death has taken place for the redemption of the transgressions that were committed under the first covenant, those who have been called may receive the promise of the eternal inheritance" (Hebrews 9:15).
The promise of eternal inheritance has not been fulfilled at this point in time. The promise is there and God always keeps His promises, but eternal life is given at Judgment Day. "Do not marvel at this; for an hour is coming, in which all who are in the tombs will hear His voice, and will come forth; those who did the good deeds to a resurrection of life, those who committed the evil deeds to a resurrection of judgment" (John 5:28-29). Thus, it doesn't matter if some are counted righteous before Jesus' death and others are counted righteous after his death, all the righteous will receive the promise of eternal life at the end of the world when Jesus returns.