Quite understandably, this lacklustre entry wasn't quite enough to make a good Game Boy debut on English shores, but to improve it, they did something completely random and, well, awesome.
To my knowledge, there's never been any kind of official explanation behind why this happened. Did Nintendo request it? Did Hudson request it? Did Wario request it? Given how Hudson would later develop the Mario Party series of games for Nintendo, one can assume the two have some form of decent working relationship, though trying to make some kind of connection between Wario Blast and that would just be rampant speculation on my part.
Due to the larger logo, Wario Blast shifts the menu and copyright jargon towards the bottom.
The Super Game Boy border rather half-heartedly throws Wario into the crowd, where palette limitations render his trademark yellow hat in purple. He also sported this look in the NES version of Wario's Woods.
The Japanese version features a brief story if you wait long enough at the title, but it was excised from the English release. See the reference page for a transcription and translation.
The "#PLAYER" text in Battle Mode is changed to "PLAYER#" for grammatical correctness in Wario Blast.
Wario Blast naturally adds in the ability to play as either Wario or Bomberman after selecting Normal. GB just skips straight to the first round. Bomberman's head is carried over from GB's title screen, while Wario's mug appears to be a small edit of his "Vs. Boss" graphic.
Art changes. The originals are retained when playing as Bomberman, of course.
Yes, it's really Wario in the game. In the original you would face a variety of slightly different Bombermen, and that is true when playing as Wario, but playing as Bomberman in Wario Blast means you fight clones of Wario. Weird stuff.
The Western edition, thanks to the two character choice, tinkers with the password system. Wario's passwords will be the reverse of Bomberman's passwords, and vice versa (Bomberman's password 4696 becomes Wario's password 6964). Bomberman's passwords in Wario Blast will work just fine in Bomberman GB.
The item explanation sequence requires two boxes of text in the Japanese version, but can fit snugly into one in the English version. The Japanese version always has a line reserved for dakuten and handakuten markers, hence the empty space between lines.
One can imagine the text between each world is different. I haven't translated any of it, but I don't expect anything groundbreaking. The Black Bomberman that chases the player (pictured on left) is replaced with Wario if you're playing as Bomberman, or Bomberman if you're playing as Wario.
The second-to-last art change.
The final art change! The Black Bomberman seen pursuing Bomberman during the intermission warning screens is replaced in the ending with Wario. Wario only has one animation though, whereas Black Bomberman has animations for both intimidation and fright. The ending still focuses on Bomberman for the entire credits sequence regardless of which character you play as.