When you work with CPP only, you loose the ability to do blend. You can code the “event graph” part of the animation blueprint, but not the animation graph.
What you can do instead is create a state machine that blends between 2 montages that are coming from a variable and you set those up in CPP. You can also do curve based animations.
Check out Advanced Locomotion System on marketplace, look for ASL v3 CPP on google and you find sample codes for both.