Take a group of 10 biracial people and you're likely to find 10 different hair textures, which is why the term biracial hair is so misleading. You can also take a group of 10 people whose parents are all of African descent and find hair that's just as varied. When it comes to black hair, particularly in places around the globe where the slave trade figured prominently, there is no one texture.
Saying that someone has biracial hair or mixed hair is often a catchall way of saying that their texture is neither straight nor coily, but somewhere in between. However, due to the way genetics works, children of interracial unions may also have hair that's entirely straight or very kinky -- not all biracial people have hair that falls into perfectly glossy ringlets. The term biracial hair can therefore be a misnomer.
You may find it more descriptive to talk about hair in terms of strand thickness and curl size instead. Some hair contains fine strands, others are coarse and still others are thick or thin. Curls range from pen-spring size to fat marker size. Just saying biracial hair may conjure up specific images in some people's minds, but it's often a too-simplistic way of defining loose curls, particularly when biracial people aren't the only ones with this texture.
In addition, due to some lingering issues in the black community, this term may also serve as a covert way of describing "good hair," a phrase that still creates a bit of animosity.
Biracial hair is really no more a specific texture than curly hair. Curls are so varied, even among members of the same family, that using this phrase isn't as descriptive as it could be. Biracial people have their own unique textures. Labeling hair, just like labeling skin color, will probably never go away, but choosing to describe the hair of people from such diverse backgrounds can never be done with just one phrase.