Thanks, Chris.
To be honest, I'm not in love with the methods discussed in that old thread, either. I think Sam's original comment, "As for me, I would much rather recreate the effects from scratch than to try and remove the black ..." is the only way to guarantee good results. There may be work-arounds that are somewhat effective in specific cases, but it's just not that difficult to recreate images like these from scratch so I think one's time is better spent doing that.
IMHO, while the request can be stated so simply, the simple wording harbors many complexities and concepts that aren't stated and which may vary from case to case. For example, how do you treat the very dark blues in the "atom" image? Should the very dark blues be regarded as blues (and retained), or blacks (and removed / turned to transparency or white)? Even if you figure out how to make this decision, then you have to figure out exactly how to make the transition between these two different approaches to tweaking the two areas so the transition looks smooth and natural.
Tom M