I can absolutely guarantee, promise, assure you and swear up and down that
hydrogren peroxide does NOT make your ultimate cyanotype print one
scintilla darker, richer, brighter or anything-er. It just oxidises to
the blue immediately that the print will reach in air on drying in a day
or two. My students refused to believe this (nor were they the only ones);
so we tore a print in half, "intensified" one, let the other dry naturally
& combined. QED.
I have, however, not tested the hydrochloric acid as precisely. My sense
of it is that the blue returns to its former shade on getting a rinse and
then drying, but perhaps there is some slight change. It's also possible,
I suppose, that some cyanotype formulas would react differently.
There's a lot on this in the alt-photo archive, as well as cyanotype
toners and a GREAT DEAL else.
Judy