I want to know if you can just have the scapular blessed or if you have to be enrolled to receive the associated graces.
From the Carmelites’ official scapular catechesis:
Is it necessary to enroll people in the Scapular Confraternity for them to share in the spiritual benefits attached to the scapular?
No, those who wear the scapular out of devotion, practice the Order’s spirituality, yet who have no formal association to the Order share in a spiritual affiliation to the Carmelite Order. It gives them the assurances of the graces pertinent to this sacramental.
So the answer is no, you don’t have to be enrolled.
However, if you can at some point get enrolled, for example during a visit to a Carmelite monastery or shrine, or at the World Apostolate of Fatima (Blue Army) shrine, it’s a nice thing to do.