Actually, it's fairly rare to see a great player blocked.  Think of Miguel Cabrera being moved to the OF, Pujols going between 3B, LF, and 1B, or even Bonds going to LF because of Van Slyke (actually, Van Slyke came the year after Bonds came up, but the switch was easy enough).

Usually the guys that are blocked are first basemen, especially lefty throwers, who are too immobile to play the OF, and especially in the NL because there is no DH.  I suppose it happens with catchers too, but the frequency of great, can't miss, strong bat catchers being developed is pretty rare.

With McCovey and Cepeda, both were with the big club, but there was a lack of playing time because Cepeda, who was most definitely the better OF, didn't want to play LF and was quite vocal about staying at 1B.  McCovey was the better hitter, but he was an absolutely terrible OF.

Alvin Dark seemed to go along with whatever Cepeda said because McCovey kept quiet and Cepeda complained all the time, so McCovey, who was ROY at age 21 in 1959 (Cepeda was ROY in 1958 at 20), didn't become the regular starter until 1963 at age 25.  Even in 1963, McCovey was the starter in LF, not 1B, and this continued in 1964 even though Stretch was injured a lot that year.

Finally, in 1965, when McCovey was 27, Cepeda was injured all year and McCovey became the regular 1B.  Cepeda was traded very early the following season.  Unfortunately, he was traded for Ray Sadecki, which was generally considered the worst trade in Giants history until a certain ill-tempered catcher came over from the Twins...

Anyway, McCovey lost, I would say conservatively, a good 40 HR and 750 AB by not starting early in his career.  McCovey was the better player, but Cepeda was louder and had a better batting average, and he insisted on playing first even though the team would have been far better served by having him play OF.

The trade was considered an unmitigated disaster, but in those days, the Giants had farm system talent coming out of their ears.  Compared to today, it is absolutely crazy to think about.  And the talent were all fine-hitting position players (except for Juan Marichal and Gaylord Perry).  Even with the player development in full swing, though, the Giants insisted on having the worst hitting SS and 2B they could find.  Hal Lanier, Tito Fuentes, Dick Schofield, Jose Pagan:  these guys made Neifi and VizII look like Bon.. Burk.. ok JT Snow.  Let's not get too carried away.

This wasn't the first trade the Giants made to get some pitching, either.  Flippy himself was traded after 1963 for a few players, the centerpiece being Bob Shaw, who actually had a great season in 1965, but that was about it.

Cepeda seems like a very cheery "Beisbol berry berry good to me - have a Cha Cha bowl!" kind of guy now, but he was something of a big whining complainer back then.  If anyone is wondering why McCovey is so beloved, even more than Mays back in the day, it might have something to do with him having the hitting skill (ok almost) of Bonds and the cheerful, polite personality of JT Snow.

Spilman was brought over to be a pinch hitter.  Aldrete was always going to be an OF.  He only played 37 games at 1B in 1986.  The Giants absolutely knew he would be the starter, and he did play 102 games at 1B in 1986.

No one was going to block the Thrill.  And after 1985, there was no one to block him anyway.  David Green?  Broken down Dan Driessen?