Grob opening on black?

That is the Borg defence. Resisting white with this is futile. 

if white having the first move can't manage to get it to playable, then it stands to reason black won't either. just make sure to not fall for tricks.

Grob is "playable" its just being -0.5 as white is not exactly enticing

the Borg is about +0.8-0.9ish Basman often plays it as a strange gambit which is not sound but surprisingly better than you would expect. He has a win  agaisnt Speelman with the Borg.

What's enticing, despite being -0.5, is your opponent being completely out of book from your first move. Throwing the rule book out the window. My Grob Storm system (https://www.chess.com/forum/view/chess-openings/grob-storm-system) takes the sentiment to its logical extreme. Chaos is a ladder, as the character Littlefinger mused on the show Game of Thrones. Dare to climb it. Dare to enter the Grob Storm.

As either white or black, too!!

Mate, you are rated in the 600's...how much 'book' do you think your average opponents have? Is your strategy working for you? I bet you could gain 400 rating points at least if you stop playing such dubious stuff. 

I can give you a demonstration, if you'd like. Just send me a challenge, 30 minutes time control (or 10 if you're busy). Rated or unrated, up to you.

🤔 Trolling? 

Ok, you won both games (https://www.chess.com/analysis/game/live/7496474759 https://www.chess.com/analysis/game/live/7496555797), but you are much higher rated than me and the system is still in development. It works for me, it may work for others.

Of course with such a rating gap the result should not be in question regardless. Playing 1.g4 and 1.g5 to get a weird position is one thing, but what you are doing with your king is just nonsense. 

Regardless of what we think about the Grob Storm, I really appreciate you giving me a chance to show you how it works. Maybe in a few months/years I'll be able to beat you utilizing it

I have every confidence that in due course you will demonstrate that what the best players in the world have been doing for 150+ years is not the best way to play 👍

The idea of moving the king up is that he can be a powerful attacking piece, especially as we see in pawn and king endgames. It is also highly unusual and likely to throw your opponent for a loop.

I see you've been doing your research. The king is a close-ranged piece so it's best near the middle, closer to all the other pieces. I have no idea why its so rare to see your opponent's king near the middle of the board. They like to keep it on the sides. I guess most people just aren't that good.

https://play.chess.com/Q3nxi

This guy got grobbed of some rating points.

https://www.chess.com/analysis/game/live/7503839623?tab=report

Ok correct if I'm wrong, but I tought that the grob is the worst first move possible by white, I'm pretty sure its losing objectively. So as black, it would be even worst...

Rating points are the single most important thing about the game of chess. It's why we all play, after all!