I ventured the Underdark up to the Forge. I then switched into the Shadow Lands and am now at the Inn, where I received the light blessing.

Rather than going to the tower via. ShadowLand route, will the game have any issue if I backtrack and go to the tower via. The underdark passage?

Thanks

If you leave the Shadowlands for any of the previous chapter 1 zones (Underdark, Crash site or mountain pass) you will loose light blessing buff with no way of getting it back.

But other than that there are no other issues you should be able to do the Shadowlands quests.

Edit
If you have just arrived at the inn and got the buff I would suggest going back to a save just before getting the buff. That way you can do the Underdark and then get the buff when you return to the inn in the Shadowlands. The buff is very useful saves the need to equip a moon lantern.

you don't actually lose the buff it just gets hidden

going back to understand doesn't make a lot of sense unless you are too low a level and want to finish up quests

the game will have storyboard issues with you leaving the area... how much that mucks things up will depend on how you romance and other options you went with

Nice to know the buff is still there. Not that that was a problem for me on my first run thorough, I had already been back long before I found the inn, just before I finished the zone.

I have completed the game twice, the first time I went back, not to complete any quests, I had done both the Underdark and the Mountain pass. I went back to visit every vendor looking for a sword that went missing from my inventory. I had no storyboard issues I was able to complete all quest lines. In fact I more than my second game when I failed to save Roland.

As for romance in both playthroughs I said no to all romance options except Karlach who was the NPC I wanted to romance and I failed in both playthroughs.

But just because I did not have storyboard issues, does not mean they can't happen. I agree with ussnorway don't travel back unless you really need to.

I did exactly as you are describing in my first playthrough; and had no issues. I did lose a buff but was able to get it back and search the shadowlands. I think I got it back by calling the fairy, or speaking to the cleric, or something. I can't recall exactly how, but know that the buff did return.

Shadowlands, received buffs at the inn, grymforge, then back to shadowlands, no issues; perfectly solvable once you get back.

the buff from the fairy is different to the cleric buff but yes they are both linked to the shadow story

the fairy can be reappied [and does need to be before reaching the tower] but the clearic one doesn't

I did both and found the way above easier, but the underdark way was more amazing. Its shorter, if you dont count the duergar, you should always go up though, cannot miss the temple.
The difference is the way you find the lantern.