What shminux means is that, even though the Schwarzschild metric was derived early as you say, its physical interpretation as a black hole was not understood till much later. According to Wikipedia, it was not until 1958 that the Schwarzschild radius was identified as an event horizon, a surface which causal influences could only cross in one direction. It was also in the 1950s that the maximal extension analytic extension of the Schwarzschild metric was constructed, and it was not till the 1960s that these results became widely known and progress in black hole physics really took off (along with Wheeler coining the term "black hole" sometime around 1964).