It's been a while since I rewatched the Ninth Doctor's season, but my original thinking was that Blon is a very self-centred character who, through no fault of her own, was born into a violent family, but who also, through her own decisions, did many monstrous things and then struggled to justify them to herself and to others.
As she tells the Doctor, she was born into a family where she was expected to participate in violence and criminal activities from childhood under threat of death. Being raised in an abusive environment where you're expected to go along with your family's crimes is obviously not a child's fault; however, it's also clear that as an adult with greater agency, Blon continues to participate in and engineer monstrous acts (destroying the Earth, ripping apart a whole city, murdering innocents). When confronted, she doesn't defend her actions but rather deflects and reflects blame back on everyone else. It wasn't her fault that she was brought up in an abusive family of criminals. Aren't you just as much a murderer if you hand her over to authorities who will execute her? She can't be an irredeemable killer; after all, she spared a woman and her unborn child even though the woman could have exposed her. She really is capable of change...after all her schemes have blown up and she's facing the end.
Combined with some surface-level similarities (large family with "wonderful" and "bold" brothers, possibly married at one point, last survivor of her immediate family), I see a lot of resemblances with Maglor. Granted, Maglor was an adult by the time the Silmarils were stolen, but it's probable that a sense of family honour and pressure was the initial motivation for him to stand with his family and swear the Oath. It's a very natural response - your family has been grievously wronged, of course you're going to stand with them and seek vengeance. What is less excusable is the actions that Maglor and his family then pursue - mass murder (three times!), abandoning the people (and family) who stood with them in their initial defiance, child abduction, plain old murder. Maglor is largely passive for most of the story, going along as part of the collective, but as the end of the First Age unfolds, so does his agency. Sirion is the first time he makes a decision as an individual, to pity Elrond and Elros and to raise them. Later, he counsels Maedhros about the Silmaril in the heavens, and most notably, at the end of the War of Wrath, he argues with Maedhros for the first time about breaking the Oath. He regrets what they have done and desires forgiveness...but in the end makes the decision to murder the guards anyways.
Both Maglor and Blon are fairly self-centred individuals who "passively" go along with their families' schemes until everything goes wrong and they're left on their own; they do feel genuine regret for the life they've lived and they are capable of giving mercy to others, especially innocents, but at the same time their feelings of regret are not enough to substantially change their behaviour. They might wish that things had turned out differently, but when given the choice not to commit more evil, they fall back on their destructive habits.
The key point is that the regret very much is genuine! Blon might be trying anything to save her life when she defends herself to the Doctor, but her desire to change and to start again comes from a genuine place as the heart of the TARDIS sees and then grants. Maglor's regret leads him to do what he can to mitigate the evil he's already done - he takes care of Elrond and Elros, he genuinely tries to argue against taking the Silmarils. Both Maglor and Blon are truly capable of change, of pity and of mercy, which is what makes it all the more meaningful when they don't change. Blon was going to destroy Cardiff before the heart of the TARDIS stopped her. Maglor killed the guards with Maedhros, took the Silmarils, and then spent the rest of his life avoiding everyone and singing about his regret.
Feeling regret does not mean making a choice to change. Nor is change a singular decision, but rather a series of ongoing and cumulative decisions informed by upbringing, environment, and other factors. Blon in her first life was not brought up to be kind; in her second life, she is given a chance to grow up in a different family even though the Doctor makes it clear that it will still be her choice what kind of person she becomes. Maglor was brought up a prince in Valinor, as proud as any of them come but also brought up in a land of peace and plenty; what he chooses to do with the rest of his life is to live in perpetual pain and regret by the side of the sea.
And then of course, there's the profound irony that their feelings of regret occur in the aftermath of their atrocities. As the Doctor points out, Blon literally pleads for mercy from the lips of a woman she murdered for her skin; her decision to spare the reporter was an undeniably good thing but it's also how she's able to justify her other heinous acts because she did a good thing once. Maglor takes pity on Elrond and Elros after massacring everyone who knows and loves them and destroying their home. Is it a good thing that he spared two young boys and cherished them? Yes, it is. But he's also the one who made them functionally war orphans in the first place.
As the Doctor says, "You let one of them go, but that's nothing new. Every now and then, a little victim's spared because she smiled, because he's got freckles, because they begged. And that's how you live with yourself. That's how you slaughter millions. Because once in a while, on a whim, if the wind's in the right direction, you happen to be kind." And I feel that really describes both characters well. That's how they live with themselves. That's how they slaughter so many people. Because once in a while, on a whim, if the wind's in the right direction, they feel a pang of conscience and they happen to be kind.
