This combination can work well on a mechanical level pretty much just by remembering to make the warlock’s spellcasting ability score Intelligence, either from the outset or when the opportunity is afforded by the synergy feat chain. The bloodblade’s high HP and extended reach also pair nicely with the various features of the warlock class. If the mechanics work well, however, the flavor feels like the two classes were made for each other. In some ways, they almost feel like two sides of the same coin; the impulses that lead a character to inject themself with monster blood and make possibly ill-advised pacts with powerful supernatural beings certainly seem like they’d come from similar places. Setting the class mix is mostly a choice between being a tank and a skirmisher. More levels of bloodblade will make it easier to stand toe-to-toe with enemies and trade damage, where more levels of warlock emphasize the mystic aspects of the character and make a more mobile, stand-off combat style advisable more of the time (though melee builds are still very possible with Eldritch Scythe and Pact of the Blade). An even mix has high versatility and about the same number of hit points as a fighter or herald of the same level. [[Synergy Feats for Multiclassers#Bloodblade w/ Warlock|Bloodblade-Warlock chains and ideas]]