I strongly disagree that the bold symbols are better, even though they’re what blackboard bold was originally conceived to approximate. The reason: Bold font draws attention (ask a typographer about type color), but usually, the bolder symbols are not the ones requiring that attention. (If I read a text about real analysis, I know that the domains and codomains will be the real numbers, for example.) The blackboard bold versions stick out much less when looking at the greater composition of the page.
I respect your opinion, and agree that with some particularly "thin" fonts (most notably, Computer Modern and its variants), the associated bold symbols are too exaggerated and produce and ugly effect.
But there is certainly some dependency on the font. For example, the Baskerville used by the Publications de l'IHÉS is a much thicker font, and the bold letters flow very naturally. Look for example here
The publications in the sixties and seventies, which are scanned, are very beautiful and the bold letters mix smoothly in the text. You cannot see them "from far away" as happens with Computer Modern.
Curiously, the modern pdfs look slightly different, with apparently thinner fonts. This may be an effect of printing, that smears the ink a bit and produces slightly thicker type?
