This card largely sucks on its own. +3 will can be a butt saver against the encounter deck or critical for pulling off a difficult spell in expert, +3 fight tends to be pretty pointless on Mystics but can help them deal the odd 1 damage (2 with fireaxe or something) but it's definitely not worth losing an equivalent of a whole turn (which is what doom is) for a mediocre boost on either of these stats.
Now the first use case where BP can begin to pull its weight is at the end of an agenda. The additional doom doesn't trigger an agenda-advancement check, and when agendas cycle they remove all doom in play, so if you overload the game board with doom on these turns (with Alyssa Graham and arcane initiate also wanting to use these windows for their doom triggers) it doesn't have any negative effect on the game state. (Watch out for this possibly changing in future scenarios). For scenarios with a lot of agendas this can be really crucial to abuse. Still, it's not the most powerful and its situational that the timing will work out that you need to do these tests before the agenda cycles.
Enter moonlight ritual. Lordy, this is an epic combo. With moonlight you can load up on the doom early in an agenda and clear it before it triggers during the mythos phase. The wonders this works are not to be understated, it is not at all hard to put 6 or 7 doom on Blood Pact early in a long agenda and simply clear it later for a swing of +21 or so in tests for one action and 3exp total. It's bonkers powerful, and fairly consistent since with BP's permanent code word you only need one of your two rituals in hand to have the combo ready.
Now here's the big catch, you need to be hyper aware of what threats are available when doing this. A stray doom or two from the encounter deck, or an unexpected discard effect can have you throwing away 6 turns because of cocky overconfidence. Stay alert for what threats exist and always assume the worst, if you do you too can tap the strength of blood magic without fear of repercussion.