Legal Question in Real Estate Law in California

That depends on the terms of the easement and the related title documents for both properties. Without reviewing those, it is not possible to answer whether they will have a right to use the easement or whether they would have to use other access to the property.

I agree with Mr. McCormick, but I add that a BLM easement is a term usually meaning an easement across public land for private owners to access their property. You don't own BLM land so it is difficult to see your objection given the limited information you have provided in your post.