It depends. Anarchists of all people, should understand that involving the police is very likely to make a situation worse, not better. For example by escalating the violence or by kidnapping someone and feeding them to the prison industrial complex, a system that is likely to make the person's behavior worse, enslave and/or torture them. There are also the practical considerations like the fact that, by the time police get there, the damage caused by the reason for the call will likely have already occurred. In addition anarchists (and others who have major problems with the state) are likely to lose trust and respect for someone who calls the cops because they're rightly perceived as our enemies. Ideally there would be always be alternatives to calling the police, unfortunately in a few cases, there aren't any good or reasonable ones, and each person has to judge for themselves whether or not some particular instance is one of those cases...
Here's a couple of thoughts about one of the examples you provided in your question. So you see someone beating someone else. Why not try yelling at them to stop? If you feel able to, and especially if you have friends nearby, why not try physically intervening if that doesn't work? If it is a domestic violence situation keep in mind that there is a good chance that the person being beaten wants it to stop but also doesn't want to involve the police. From my POV there are both healthy and unhealthy reasons for this attitude. Whatever the reasons, however, why would you do something that the person who is being harmed doesn't want? You could try asking them at least...
As for Bob Black, yeah that dude is a snitch. In his case he wrote a letter to the police in order to fuck over someone he didn't like. He was not in the remotest danger, nor was he trying to help anyone else who was. In addition, most people who snitch on others *at least* have the reason (but not the excuse) that they'll serve long prison sentences if they don't cooperate. IOW they're not just doing it out of spite...
http://www.seesharppress.com/black.html BTW This doesn't necessarily mean that his ideas are bad...his ideas are bad on their own IMO.