The two previous responses to your question were absolutely 'spot-on.' Careful of those shoulders! I'll add that:
In answer to your question:
...is bruising inevitable if you hold arm balances a long time?
No...not in my experience.
Also, I am not sure what you mean by 'long?' i.e., two minutes actual clock-time, 5 minutes, 20, 30....more?
In my own practice sometimes I focus on this area for 2+ hours, holding various asanas with, and then without, props as long as I comfortably can--without strain. Though in my fifties now I have never had bruising. I may be a bit sore the next day after working intensely on arm balances...but never bruised.
Clarissa, as a fellow yogini, we women generally have weaker arms than men and therefore must work diligently to master various arm balancing asanas. Forcing the issue only leads to strain, pain and injury.
I would like to share with you the following story of one of my students:
I've had the most fortunate past experience (1990-05) to have taught thousands of senior adults while Ath Dir of the largest active adult community in Florida. I remember one student (80 years young) who was consistently aggressive and stubborn in her asana practice. She was amazingly flexible...physically 'advanced' in many asanas but not balanced in her approach...too aggressive. She was a bit recalcitrant in her demeanor as well.
During Upavistha Konasana, (seated angle pose) I saw her straining to widen the legs beyond ability....forcing the pose. I had to constantly remind her to relax and ease-up during asanas--not force them. Well, the next day (I taught classes 5 days a week, M-F)...she showed me bruising on both inner thighs! She was shocked; I was not. Seeing the evidence of her own aggressiveness manifested on her body helped to change this pattern.
That lesson was a powerful one for her.
As Inner Athlete asked: Which arm balances were you doing?
"I lost my bandhas"
(control)...now, that's funny! May I borrow it, Clarissa?
I have a sort of 'saying of the month' and this just may be it for August!