You can study the historical data for the Bloodvein here:
http://wateroffice.ec.gc.ca/report/repo ... tn=05RB003(It may force you to go through the main page, station number in at the end of the link)
If you go back over the past 10+ years you'll see that "normal" for good levels is end of May to end of June. However at least 1 in 4 years the levels are very high all Summer or a big spike at the end of summer running into Sept/Oct.
I've only been on the river in low water +/- 50cms, at that level some of the bigger rapids/ledges are too rocky in my opinion. I would think that if you are hoping to run most or all of the III's and IV's you would want to be there when the levels are around 100+, if you are comfortable in big wave trains there might not be much of an upper limit. The small rapids are generally free of rocks at almost any level, hard to say for sure but probably a lot of them are just big swifts in high water.
As Alan mentioned, fully understand that although there are about 75 rapids from Artery Lake down I doubt there is 10km of actual moving water in the entire river, the current when there is any rarely exceeds 1.5kmph.
Very beautiful river, lots of quality campsites.