It's the new DLC for Hellpoint. How does it compare to the original game? Are the new areas accessible right away or does a certain amount of data need to be discovered first?

I wouldn't know, since last time I played Hellpoint, I discovered this game-breaking, save-breaking bug as described here, and which another GOG user has also corroborated as being a real thing:

https://www.gog.com/forum/hellpoint/load_option_became_greyed_out_hence_i_cant_load_my_saved_game_and_cloud_saves_are_missing_too

...a game-breaking bug which apparently, they have never bothered to fix AFAIK.

Well, I have a good news for you my ancient friend, update 488 solve that problem apparently, I don't experienced the same as you, but probably is solved, coming soon to GOG.
PS: I said "coming" because I haven't checked Galaxy yet... Neither I will, so let's wait XD Have yet to check it out, looks good, I like what I played of Hellpoint, is a janky low budget dark souls, but it has a haet when the combat makes click for you. Hope I will play the DLC as soon as I get my PC.

As I was on my end game character when I tried it, I don't know if you need a certain amount of Data to access, but it is pretty badass. I love the content, the puzzles and the difficulty increase. It's all good stuff.

How many hours does it take to complete Blue Sun?

This is from the Discord's announcements channel:

Q: What does Blue Sun feature in terms of new content?
A: It introduces a whole new parallel dimension with access to three additional locations, in and around the Irid Novo space station. This dimension can be accessed via the breach that is located in the Observatory. Blue Sun is also going to add new weapons (including two-handed), armor sets, shields, a new “contract” quest system, cube programs, taunts and much more. Besides that, the base game is going to receive an update that adds features highly requested by the community such as inventory management and stats respec.

Q: Do I have to start a new game/create a new character to access the content of the add-on?
A: No, you can continue your current playthrough.

Q: What are the requirements to access the DLC?
A: Having collected 50% of data, which is the game’s main measure of progression. Once you do, head over to the Observatory to meet with a new visitor.

Is there a reason for the "50% of data" requirement?

That's a good question, better suited for people not me. I have yet to finish it, but it seems like a solid 6-10 or so hours worth of stuff.