This interesting British document on the G loads sustainable by German WW1 fighters pegs the highest G factor for a British fighter at 5.5G for reference, with the SE5. German fighters examined and tested by the British after the war didn't fare quite as well, with max G tending to be in the 2-3 G range, beyond which the rear wing spar would fail.
What is also important to consider is that WW1 aircraft on both sides were not capable of sustaining those G's for very long, due to the low airspeed and rather underpowered engines of that day. An aircraft sustaining very high G's bleeds off quite a bit of speed in the process, due to the increased drag.
A WW2 fighter capable of 300-350 knots with a 1500-2000 hp engine, can pull a lot of g's for a long time, before scrubbing off enough speed to be in danger. A WW1 fighter at 80 knots, with a 150-200 hp engine, would only be able to sustain high G's for a few seconds, before they lost enough airspeed to risk stalling.
The few motion pictures of dogfights of that era, presumably re-enacted, tend to reinforce this: the planes appear to make a series of brief sharp turns interspersed with returns to level or near level flight, sort of a jerky movement as opposed to the long high G turns of WW2 fighters.
Finally, there were techniques some pilots used to avoid blacking out. Major Greg Boyington, who had been a boxer before the war, reported withstanding more G's longer by tightening his neck muscles up.