I am very fond of Visconti Blue, (a little difficult to flush, but quite beautiful).
Diamine Midnight Blue is also a nice color, (not quite as nice as Visconti), but a little easier to flush out and a lot cheaper.
PR American Blue is very nice - the closest match to Penman Sapphire I've found, but you have to flush your pen after each fill - it is pretty saturated. Some people recommend dilution, but I do not, because when you dilute an ink, you also dilute the chemicals in it that protect against mold/SITB; provide lubrication and flow enhancement. So, unless you want to augment those chemical additives yourself, (you can), I would not recommend dilution. I'm sure others will disagree.
All the best,
Edited by corniche, 06 September 2013 - 00:17.