BOSS won't cause inflammation but it will aggravate any sort of inflammation because it is so high in Omega-6 and lacking in Omega-3.
Flax contains both so is the safer route.
This link is four years old but is a well-written article regarding feeding BOSS to horses. BOSS: Black Oil Sunflower Seeds for Horses
I won't touch the stuff, I prefer Omega-3 Horseshine which has a natural balance of -3 & -6. I especially won't touch BOSS as I have two horses with metabolic issues and excessive amounts of Omega-6 are not healthy for them. The EMS exploded the hock arthritis on one horse and I have no doubt BOSS would aggravate than inflammation.
Conversely, I am acquainted with one or two folks who give their horses injections for arthritis and STILL won't take the BOSS out of the diets. They are convinced BOSS is the be-all-end-all to their horses good health
Since no scientific studies have been done, to my knowlege, nobody knows the long-term good or bad effects of BOSS. Some are willing to take this cheap route, others of us are not.
For anyone that does by BOSS:
1. DO NOT buy the bird seed that says "bird kote" on the bag.
2. I have heard not to buy the striped hulls but I don't know why.
3. It is better to spend the extra $$ and buy the hulless seed but then you might as well buy the Omega-3 Horseshine and do that portion of the diet the right way
Hope this helps answer your question