Is ZH/P being a D too harsh a defensive grade for Blissey?

Agreed. I don't get the ranking either.

It depends on who you are using as an attacker and your dodge strategy/skill. For example, ZH/P is one of the most challenging movesets for a Machamp attacker, with all Blissey attacks being SE and all dodges being critical. In contrast, ZH/P is probably the easiest moveset for Exeguttor with it resisting all moves. I prefer to use B/SE Tyranitar and dodge everything. Bite is fast enough to have no problem hitting dodges, and Tyranitar resists everything except DG. I can make it through ~3 P Blisseys in a row, 2 HB Blisseys in a row, and not much more than 1 DG; fitting with the site ranking.

I agree, I think the ranking grades are based on strictly damage output not taking dodging into account, i.e. Not factoring in real practice variables. Assuming a player dodges HB but gets hit by psychic:

HB = 150 * 1.25 (stab) = 187.5 / 4 (dodge) =~~ 47

P = 100

Grading Blissey's moves is like trying to split hairs. Just stick any Blissey in any gym. It's all about her stamina/defense. Save one with Pound/HB for attacking if you want.