The alternate capstone descriptions says, When a character reaches 20th level, the following new abilities can be selected instead of the standard 20th level class ability which would normally be gained. In some cases, a capstone specifies what ability it replaces. A character can’t select an alternative capstone if she has previously traded away her class capstone via an archetype."
The last part is the the more important part, saying you cant take an alternate capstone if you have traded away your capstone for an archetype's capstone. My interpretation is that since crossblooded doesnt trade away or replace your capstone, merely letting you pick from two bloodlines capstone and choose one, you can replace it with unique bloodline. Mainly because it says replace instead of alter or modify which is quoted as the reason why crossblooded can't go with other archetypes like wildblooded or tattooed(tho those i play with and i allow combinations like crossblooded/tattooed or wildblooded).
But based on RAW, can i trade the sorcerer capstone with unique bloodline?