Just thought I'd toss this out there and see what happens.
Casting: The Mystic Blade Magus may combine spheres and talents to create magical effects. The sphere magus is considered a Mid-Caster and uses Intelligence as his casting ability modifier. (Note: All casters gain 2 bonus talents and may select a casting tradition the first time they gain the casting class feature.)
This replaces the spells class feature.
Spell Pool: The Mystic Blade Magus gains a small reservoir of energy he can call on to create truly wondrous effects, called a spell pool. This pool contains a number of spell points equal to his level + his Intelligence modifier (minimum: 1). This pool replenishes once per day after roughly 8 hours of rest.
Magic Talents: A Mystic Blade Magus gains 1 magic talent every time he gains a caster level. This does not stack with caster levels gained from other sources.
Arcane Pool: A Mystic Blade Magus does not gain an arcane pool. Instead, any time he may spend an arcane point, he may instead spend a spell point to achieve the same result. Though they generally do not have the ability to enhance their black blade's enhancement bonus beyond what it normally is. They Can, however, use other enhancement talents freely upon it and treat their magus levels as full caster levels when they do so.
This modifies the arcane pool class feature.
Black Blade: At your first level as a Mystic Blade Magus you gain one masterwork weapon of your choice and the Focus Casting (black blade) drawback. However he does not count this drawback towards how many drawbacks he has for gaining boons or spell points, and cannot buy off this drawback.
Arcane Potency: At 4th level, a sphere magus adds half his magus level to the number of spell points his Black Blade has.
This replaces Spell Recall and Greater Spell recall
At 3rd level, the Mystic Blade Magus focus reveals its nature as a powerful sentient weapon called a black blade, whose weapon type is chosen by the magus (see the black blade sidebar). A magus with this class feature cannot take the familiar magus arcana, and cannot have a familiar of any kind, even from another class.
This ability replaces the magus arcana gained at 3rd level.
Black Blade Knowledge: At 7th level, The Black Blade has its own magical abilities. when the Magus rests to regain spell points, they may decide what magical talent their black blade 'knows' for the day. Any spell points used for this effect must come from the Black Blade itself. However 13th level or higher mystic blade magi may use the Transfer Arcana ability to transfer one talent their black blade knows to them for one minute as they can transfer spell points.
This replaces Knowledge Pool.
Masterful Magic (Su): At 19th level, whenever the black blade uses its Black Blade Knowledge feature, It may choose two magic talents or base spheres to gain instead of one.
This replaces greater spell access.
Changes to the Black Blade.
Lifedrinker: The effects of the black blade become this.
The Black Blade regains 2 spell points
The Magus gains 1 Temporary spell point
the magus gains a number of temporary hit points equal to the black blade’s ego (these temporary hit points last until spent or 1 minute, whichever is shorter). The creature killed must have a number of Hit Dice equal to half the magus’s character level for this to occur.
Arcana and archetype notes
Born to the Black Blade: If you use an archetype like the Mystic, which removes your ability to use Spell Combat, you can choose to count abilities like Energy Blade, Spell Attack, and Spellstrike as spell combat for the sake of this ability
Black Blade Thirst: Not specific to black blade, but you may treat Vampiric Strike from the Death Sphere as Vampiric Thirst for the prerequisites.
Mystic: You may combine the Mystic Blade archetype with the Mystic, however you replace Mage's Potency with Arcane Potency from the Sphere Magus.
Some design worries
1: making This version of the Black Blade slightly better than the normal black blade. Mostly I did this because of the fact that Sphere magi (and especially Mystics) rely far less on needing to have an empty hand due to not being guaranteed to need somatic components.