Clerics of Selûne can cast Bless from a scroll (which I am assuming is in prayer form) created by Clerics of Shar. That doesn't really match the concept of channeled magic.
Think of scrolls as a requisition form; a Shar cleric might produce a spell scroll for a particular spell, and thus needs to expend the energy required to open the conduit to her deity and draw that power - but she doesn't actually evoke the effect, just imbues the form of the 'request' onto a prepared scroll. The scroll itself doesn't contain any essence of Shar, and it doesn't link directly to Shar either - a non-cleric, or someone without a divine connection, couldn't do anything with it (in 5e rules). It's just the magically formed structure of a particular divine request, reprepared so that it doesn't drain the user at all to open their channel in that way.
If the scroll gets stolen by a Selunite, it's not a scroll asking Shar for magic, it's just a scroll pre-prepared to allow a cleric to open their divine connection in a particular way, and make a particular request, without putting any strain or drain on their mind/body. So the Selunite can use this scroll to open her divine connection to Selune, in the form that the scroll describes, and request a Bless from her
own deity, without taxing her mind or body in the process, and without preparing in advance to ask for it.
