Went to the range with 5 .357 rounds each loaded at 10.0 and10.5 grains of True Blue, 125 grain jsp, the starting load is 10.4 grains. 1/5 shots did not register on my chrono due to bad lighting in both tests. The 10.0 grain load was like a scatter gun and mv was in the around 1180. The 10.5 grain loads were between 1225 and 1240 fps with decent accuracy (I was shooting kneeling in the snow so they may prove to be very accurate). So this powder does appear to be a good possibility in .357 magnum. I'll keep ya posted. BTW, I was using standard pistol primers, can't find any magnums anywhere right now. I figured 10.5 grains should ignite fine with standard primers.
NOTE.
Only 4/5 shots in each test registered on my chronograph due to bad lighting conditions (yes I could have deleted the one shot and got an SD, but I was in the snow wearing Converse Allstars so I was in a hurry). The 10.5 grain group was decent but I was shooting off hand, I can see it being a very good load. Trying 10.7 and 11.0 tomorrow.

This was out of my 640 Pro with a 2 1/8" barrel.

__________________
Still got the shovel

I hope the the weather improves for you its been nice here in central OR for a while but that will soon change.
I would like to to try True Blue but nobody stocks it locally.
I have good results with Accurate #7 with jacketed bullets and magnum primers in a Ruger sec. six w/a 2.75" barrel, I like the mag. primers for colder temperatures but standard primers also does well with powder.

Oddly enough its the ONLY powder I could find locally suitable for .357.

__________________
Still got the shovel