Strength for Blood will still be point per point better for you than any other stat so stacking strength will always be the best gain.
Personally, the only time I'd start gemming ArP is if I'm around 30% with my gear, and I have an ArP trinket. Short of that, some people do still gem ArP but I find Strength to be better in this situation. Some people will say "You're full of crap!" but ArP simply does not affect around 15% of your damage. If 15% of that damage was physical and we were a pure physical class when it comes to being Blood, then I'd say go ArP. Unfortunately, Strength will always be your 'best' stat to get.
Get one of the trinkets, then you can consider switching to ArP gemming.