Okay, about Big Blue, I think it "should" be legal in the sense that I do not believe the current evidence suggests it's unfair. The stage would be the least fair counterpick if allowed though, and it has several features that I don't think anyone likes. Sometimes you really can't see what's beneath you which shouldn't make you die but does put you in bad positions, and being killed by shield pushback is just dumb (if shield pushback puts you on the track, you are all but guaranteed dead). I know certain people are never going to listen about this stage, and I could see very realistically the stage proving broken eventually. Of course, I find matches there right now fair enough... If I were going to go all out for a stage that won't win, it would actually be Rumble Falls (seriously, there is nothing wrong with Rumble Falls). However, both Rumble Falls and Big Blue are just not battles that can be won.
If you're really curious, this is my dream stage list that I would pursue if politics weren't a concern at all. This is probably the most liberal stage list you will ever find that isn't essentially "allow everything"; I really love this game's stage list. This is going to be so controversial that I wouldn't even use it if I were hosting a tournament myself, just so you know.
Starter
Battlefield
Final Destination
Delfino Plaza
Yoshi's Island (Brawl)
Halberd
Lylat Cruise
Smashville
PictoChat
Pokemon Stadium 1
Counterpick
Luigi's Mansion
Mario Circuit (I have a few reservations involving how easy the walk-offs are, mostly okay with it)
Rumble Falls
Norfair
Frigate Orpheon
Pokemon Stadium 2
Castle Siege
Distant Planet
Green Hill Zone (I have a few reservations involving how easy the walk-offs are, mostly okay with it)
Yoshi's Island (Melee)
Jungle Japes
Onett
Corneria
Rainbow Cruise
Green Greens
Big Blue (I have a few reservations involving how this may end up being way too powerful for some characters with the radical structure, mostly okay with it)
Brinstar
Maybe Counterpick (I'm still not sure about these stages to the point where I am a bit uneasy about them being legal)
Mushroomy Kingdom 1-1 (a very weird stage that I have extremely little experience on; the fact that all deaths here are super low makes me uneasy, and Snake seems extremely overpowered here. I still am not completely sure about it though)
Pirate Ship (the water here is basically incredibly stupid and seems to lead to really degenerate gameplay, but it's hard to ignore lots of tournaments that were fine and allowed this stage)
Port Town Aero Dive (I go back and forth on whether the cars are too much or are acceptable. I had been drifting toward thinking it needed to be banned, but some recent stuff I've seen about it has me starting back the other way now)
Skyworld (I'm not sure if Olimar is actually capable of handling this level or not, and it has an awful lot of teching. I had some matches here recently that were surprisingly not that bad and made me rethink my "this stage is obviously completely broken" position, but I'm not totally sold yet)
Doubles only counterpick
Bridge of Eldin (the power of the walk-offs and the spam is reduced in doubles, and any time concerns you may have had disappear)
Summit (the loop isn't abuseable in doubles, and seriously nothing else about this stage is terribly abuseable. The fish is extremely predictable and a dumb reason to ban it).
Doubles unsure counterpick (could be fair or not; I'm not sure about it, not worth it in practice because people don't like too many doubles only stages)
75m (no joke, explained shortly below)
Hanenbow (neither of these stages is worse than the other; 75m is a bit misunderstood. It is VERY similar to Hanenbow in terms of its problems with soft loops and campy small platforms; the main differences are that 75m has some relatively unimportant hazards and walk-offs with smaller platforms and a very questionable "two long platforms almost on top of each other" area while the ground on Hanenbow is constantly moving when hit and generally isn't flat and slow characters don't get ladders to use to ascend which both make Hanenbow a bit campier)
Banned always
Mushroomy Kingdom 1-2 (the upper part with the ridiculous ceiling is ban-worthy no matter what, it leads to a strategy that causes deaths so low that this stage produces essentially random results if one side chooses to pursue it)
Spear Pillar (tons of teching, the loop is way too easy even for doubles, and I can't accept reversed controls as a fair hazard)
WarioWare (random stars/mushrooms are unacceptable which is a shame because otherwise this is a very good level)
New Pork City (too huge, too loopy)
Mario Bros. (loopy, gamebreaking hazards that lead to essentially random results as both sides abuse them, too much focus on teching)
Flat Zone 2 (the lion tamers are too deadly of a hazard and not always avoidable, the stage is overall too strong of a counterpick for certain projectile spammers, otherwise it would be a good counterpick; this level is a very cool level and is a huge loss to the game)
Shadow Moses Island (too much bias for powerful vertical kills which pretty much gives Sonic among others 0-100 against most of the cast, too good for walk-off abuse and wall infinites, extremely campy, extreme focus on teching; this stage is like Venom from melee insofar as it's not simple in how it's radical if you don't consider the walls a problem, but it's just a very dumb and very terrible level)
Temple (too huge, too loopy, too much teching)
However, politics means I don't fight for Rumble Falls or Big Blue for singles or for Summit for doubles or for any of the stages I really am not sure about since I think they all really need honest investigation more than anything, but most of them are ridiculously controversial in addition to being questionable at best in terms of fairness so I just resigned to not getting to see about them. I only posted this because you were probing for what I actually thought; don't take anything from it in terms of what positions I want to work for because pursuing a list like this is just a good way to never be able to get anything you want.
Back to Green Greens, about the exploding field, I just don't think it's broken. It's like Ness's yoyo glitch from melee except I don't think you can make those hitboxes actually move. It should be obvious enough once it's invoked so you just don't go into it once it happens. It's one of the few mechanics I actually don't understand that well though so any exploding field experts chiming in would be awesome.