When it was originally released in Japan as Meta Fight, Blaster Master failed to make much of an impact. In the west, however, it was retooled slightly and given a new story about a boy and his pet frog, and became a cult classic that spawned multiple sequels - several of which ended up not even being released in Japan.
On the surface, Blaster Master is a bit like Metroid, but with its own unique twists - just like Metroid, you travel through a giant maze-like world, defeating bosses and collecting new power-ups to access more areas. Generally, this is done in your armoured tank, the SOPHIA, which can eventually shoot, jump, float, cling to walls and more.
The twist is that you can actually exit the tank and run around on foot as Jason, its driver. Jason is pretty much powerless in the overworld, but there are special small gateways that only he can enter, in which the game turns into more of an overhead shooter. These are usually a bit more straightforward than the rest of the game, but as you get further into the adventure they have some tricks up their sleeves. All of the game's (quite difficult) bosses are located in these areas, so you have to make sure you explore them fully.
Interestingly, in these areas, your gun's strength is tied to Jason's health, so getting hit will actually weaken your gun as well. Thankfully, you have a close-range grenade attack to fall back on, but of course it's always smarter to avoid taking damage to begin with.
A big gripe with the original Metroid is that it can get very confusing to navigate, but thankfully, it's a lot easier to get around in Blaster Master. Each area has its own distinct graphical style and there's an almost complete lack of nearly-identical rooms. The areas are also quite a bit smaller, and exits to other places are marked by big, obvious gateways, so you know that you should turn back if you haven't yet beaten the current area's boss.
Blaster Master is quite the large game, with 8 unique regions. That, plus the fact that each has several Jason-only overhead sections, means it can take quite a while to get through the whole thing - this is actually one of the longer NES titles. This means it's yet another great fit for the 3DS Virtual Console's save state feature, as you can just save and quit wherever you want (or save before those difficult bosses so you don't need to backtrack if you suffer a game over).
As mentioned above, the game's graphics are quite good, with a different look for each area. They're accompanied by a soundtrack that also has different music for each area, and a lot of it is quite catchy.
Despite all their attempts, Sunsoft never quite managed to top the first Blaster Master game, which is evident by how many people seem to be unaware sequels even exist. It's still very much playable even to this day, and perhaps a gem you might have overlooked until now. This is well worth a look.