Rename your current Blessing of Sanctuary macro to BS.
Make another macro called Set BS put this in it:
/run local n=UnitName("target") or "player" if not InCombatLockdown() then EditMacro(GetMacroIndexByName("BS"),nil,nil,"#showtooltip\n/cast [@"..n.."] Blessing of Sanctuary") print(Sanct target set to: "..n) end
Target your healer and hit the Set BS macro at the start of the match. Then your BS macro will be set to cast at them without needing to manually update it.