is my pokemon black team good?

Sceptile is fine. Just because it has only one elemental type rather than two does not make it inferior. What makes Blastoise or Machamp an exception? In fact, there are advantages to having only one type. Depending on the type though, a Pokemon can have more resistances, or more weaknesses. A Pokemon like Charizard is most vulnerable to Rock-type attacks since both Fire and Flying-types are weak against Rock-type attacks.

So, Gengar is a Ghost/Poison-type, Machamp is a Fighting-type, Blastoise is a Water-type, Sceptile is a Grass-type, Charizard is a Fire/Flying-type, and Umbreon is a Dark-type. Good mixup. It doesn't appear that any of your Pokemon have similar weaknesses, except for Gengar and Machamp. They both are weak against Psychic-types. Well, more specifically, Poison and Fighting-types are weak against Psychic-types.

Now, a good thing to do is give each of your Pokemon a type of move that can counter against other Pokemon it is weak against. For example, since Blastoise is weak against Grass and Electric-types,it would be a good idea to give it Earthquake for Electric-types, or Ice Beam for Grass-types. Both are TM's Blastoise can learn.

Did this do more than help? BTW, do you know what EV's are?

Sceptile isn't the best Because he's Only grass type Its best to have Pokemon with multiple types So That you have an edge other Than That its a pretty okay team

level up to at least level 80 and it'll be a great team