One common theme is the idea of finding acceptance. The OC often has to find their place in the RWBY world, and Blake is a part of that journey. For example, in many stories, the OC may be an outcast like Blake was, and they bond over their shared experiences of not quite fitting in.
Another theme is the power of friendship. Blake and the OC usually form a strong friendship or something more. They rely on each other in battles and in emotional situations. A lot of fanfictions will show them training together and growing stronger both as individuals and as a team. It's about how they support and encourage each other to be better.
Love and relationships are also quite common. Whether it's a slow - burn romance or a more immediate connection, the relationship between Blake and the OC is explored in depth. Some stories focus on the cultural differences between them (since Blake has her own unique Faunus heritage) and how they overcome those differences to be together. It's all about the connection, the attraction, and the growth of their relationship.