The blessing of the scapular (and the recipient, in reality) is inconsequential to the question.
The enrollment in the scapular does not have official preconditions. However, you are making a promise to live a pure life in dedication to the Blessed Mother. So, from the onset, you are making a farce of a blessed privilege by knowingly promising your purity and devotion while in the state of mortal sin. More amazing would be that you are receiving the scapular from a priest who could certainly absolve the sin prior to enrollment. It approaches the level of sacrilege but is not because the scapular is a sacramental and not a Sacrament.
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