As an action, you can imbue one weapon that you are holding with positive energy, using your Channel Divinity. For 1 minute, you add your Charisma modifier to attack rolls made with that weapon. The weapon also emits bright light in a 20-foot radius and dim light 20 feet beyond that. If the weapon is not already magical, it becomes magical for the duration.
The shadow blade spell description (XGtE, p. 164) says:
You weave together threads of shadow to create a sword of solidified gloom in your hand. It counts as a simple melee weapon with which you are proficient. In addition, when you use the sword to attack a target that is in dim light or darkness, you make the attack roll with advantage.
If my Devotion paladin conjures a shadow blade and attacks a target in the dim caverns, I attack with advantage.
If use Sacred Weapon on it, it now emits bright light. Do I lose all advantage gained with the shadow blade?