The only logical answer I can figure is that Blondie picked up where Tuco left off where Bill Carson was talking about the gold and the cemetary. BC told Tuco the name, but when Tuco went to fetch him some wata', BC told Blondie the name of the grave. When he was telling Blondie the name of the grave, he probably just spit out the name in a whisper, then died. Blondie caught on to what BC and Tuco were talking about before he got there. He could have also been on the side of the wagon, listening to Tuco's threats and heard him mention the gold coins when Tuco was shaking BC up in the wagon. It's also possible that Blondie got wind of the gold coins right from the beginning and was searching for it just as Angel Eyes was in the start of the movie. Does any of this make sense to you? I know it's a long explanation, but I'm just trying to explain it the best way I can and thoroughly - that's all.

Hope this helps.