As far every mage that summoned demons was blood mage like uldred or avernus and malcolm hawke and many others so i doubt that it is accident. Even codex says that.
well that is not entirely true maleficars are mages that turn into forbidden magic usually that means blood magic as it is most popular example but also if i remember correctly shapeshifting also makes you maleficar because it is forbidden by chantry.
That is the popular thing, yes, with Alistair, Wynne AND Leliana each calling Morrigan a maleficar at least once despite her not being a blood mage.
Me, I see a maleficar as the Chant of Light says it. "Foul and corrupt is he who takes the Maker's gift and turns it against his children. They will be named maleficar," or yada yada. I'm too tired to look up the whole codex right now
Essentially anyone who harms another without provocation is a maleficar, so I would see Alrik as one despite him not being a mage, and Morrigan may, in the end, qualify as one based on how the Dark Ritual turns out. Tahrone and Decimus most certainly qualified although I don't think Merrill does despite being a blood mage because she never goes out of her way to harm others.
On a side note, I'm replaying DA2 and I'm running a tally on how many offically declared blood mages I'm running into and fighting, or murder knife. I'm nearly through act 2 and I'm still at less than 5.