Just downloaded this in the U.S. I have fond memories of the first game on the NES console, after playing for about an hour I am happy to report that this gameboy color sequel is indeed shaping up to be better than the original Post your impressions here. For those of you who are interested, here's a walkthrough:

http://www.gamefaqs.com/gbc/371213-blaster-master-enemy-below...

History of the game (from the walkthrough above:)

Blaster Master: Enemy Below was released rather unceremoniously in September of
2000, after some major delays. It is the sequel to the classic NES game Blaster
Master, and features similar levels, palettes and gameplay, although it does
include some new aspects. The game is one of only a handful of dual carts,
meaning it has 2 games inside, one that is compatible with Gameboy Color, and
one that is compatible with older Gameboys, Gameboy Pockets or Super Gameboys.

The plotline of the game is as follows: Weary from battle, Jason (hero of the
game) moves far away to start a new life. Then one day, a woman he's never
seen before comes to his door, and pleas with Jason to rid a secret underground
facility of a reproductive life form that's on the loose. This, of course, is
the offspring of the organisms of the original Blaster Master. The life form
spreads fast and hollows out the image of it's old lair (hence explaining why
the levels are so similar). Jason knows what he must do, he re-activates
S.O.P.H.I.A. the third (AKA Sophia, his tank) and heads back into battle

I enjoyed the Nes original after playing it on Wii. I also quite liked Overdrive on Wiiware so I downloaded Enemy below without hesitation.
It's really nice to finally be able to play this. I never tried it on Gameboy color.
I didn't even know what Blaster Master was back then. Another cool thing, my first name actually is Jason just like the Guy in the game.