<Introductory Flavor Text also an explanation on how this can be used to emulate a 3e Warlock or something?>
So, the Classical Magical Girl. The girls that Puella Magi think they're signing on to be. These are champions of Love, Life, Hope, and Pretty Dresses. No matter how dark your setting, if even one of these unapologetically exists, that world is Noblebright.
They are generally young girls, though subversions exist in anime; there's middle-aged men and house-wives, ambiguously gay crossdressing boys, and the iconic dashing princely archetypes like Tuxedo Mask of Sailor Moon.
A Classical Magical Girl is generally Good-aligned, much like a paladin; I'll make a post about Dark Magical Girls another time.
For the record, this class could be used to simulate a Warlock as defined in 3rd or 5th edition D&D; just ignore the Henshin class feature and all that implies, lower the Prime requisite to CHA 9 and Hit Dice and Armor/Weapon Proficiencies to that of a Thief's.
For Mechanics purposes, I'll refer to this class as the Bishoujo Senshi.
The Eldritch Blast normally does Force damage. If the Bishoujo Senshi chooses at 1st level for it to do Fire, Cold, Lightning or Acid damage (or some other elemental damage type that could be commonly resisted), it does +1 damage, and an additional +1 each time the die size improves, but uses that sort of damage permanently. The Eldritch Blast may be enhanced by spells, invocations, and abilities which improve weapons, and is subject to Spell Resistance. It has a Casting Time of 1, but cannot be interrupted like a normal spell.
A Bishoujo Senshi can, at character creation, choose to channel their Eldritch Blast through a conjured cutesy wand, which functions as a mace as if conjured by a Puella Magi's Magic Weapon feature. If chosen, their Eldritch Blasts have -1 damage.
Invocations: At 1st level, a Bishoujo Senshi learns a single invocation. Invocations are equivalent to spells, but may either be cast repeatedly, or function indefinitely depending on the nature of the spell. Every three levels thereafter (at 4, 7, 10, 13, 16, etc.), the Bishoujo Senshi gains another invocation. At 1st level, the invocation chosen may be a 1st level spell from any spell list. At 7th level, the invocation may be a Cantrip, 1st level, or 2nd level spell from any spell list. At 13th level, 3rd level spells may be chosen, and so on. Once chosen, Invocations cannot be changed, unless the GM agrees that an invocation is being "upgraded", and a lesser invocation is put in the place of the upgraded one (so, if Bless were learned at 4th level, at 13th level that might be upgraded to Prayer, and Bless would be replaced with another 1st level spell as an invocation). If an invocation is not constantly in effect, it requires the standard casting time to enact.
Bishoujo Senshi likewise know a number of Lesser Invocations; these are Cantrips from any spell list, equal to their Charisma Modifier. They are otherwise subject to the same rules as Invocations.
If your game doesn't have Cantrips, give them a bonus 1-level Invocation that can't be used for combat purposes (no healing, no attacks; something like Augury or Detect Magic).