They´re not rear fanged, but they have lots of enzymes in their saliva (as all other snakes) that might show some kind of reaction.
Aglyph snakes have been documented to cause swellings due to enzyme reaction in bites. Some snakes have teeth disposed very closely to each other, acting as a duct via capilarity, thus being capable of saliva/enzyme injection. But thats not the case with treeboas.
Your case is just probably a larger blood vessel that was puctured under the skin, causing blood to flow off and form a bruise. This is very common with treeboas as they do have very long teeth that can reach deeper than most other non venomous snakes in human skin.