If an Oath of Devotion paladin uses Sacred Weapon on the Shadow Blade spell, do they lose advantage from attacking a target in dim light?

Question

The Oath of Devotion paladin gets the Sacred Weapon Channel Divinity option at 3rd level:

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?

Answer 1

Yes, mostly

Sacred weapon would create bright light for a 20 ft radius:

As an action, you can imbue one weapon that you are holding with positive energy, using your Channel Divinity. For 1 minute, [...] weapon also emits bright light in a 20-foot radius and dim light 20 feet beyond that.

--- Player's Handbook, p. 86

Meaning any creature within 20 ft. (and not covered/blocked) would be in bright light. Shadow blade grant advantage on attacks against targets in dim light or darkness:

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. It deals 2d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60). 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.

--- Xanathar's Guide To Everything, p. 164

But since they are in bright light, no advantage. However, assuming no other sources of light, creatures more than 20 feet from you are in dim light or darkness and so are eligible for advantage. You can then either have an absurd reach¹, or make use of the thrown (20/60) property of the shadow blade. Unfortunately the advantage from shadow blade and disadvantage from long range cancel out and you make a normal attack, but now the advantage from shadow blade is still doing something.

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1 See this question for some ideas