Grass Knot is definitely a no. It tends to be weaker than it needs to be, and would be a second Water coverage. Rising Voltage is better than Grass Knot, and is STAB.
Surf only covers types you don't at the moment, Fire, Ground, and Rock. It has a Base Power of 90, which is nothing to sneeze at, but definitely isn't comparable to Focus Blast's Power. Surf obviously does have the negative of hitting your ally in Doubles.
Focus Blast is a hard hitter and a hard misser. It's accuracy is t e r r I b l e, and you only have 8 PP to go with it. In exchange, it's really powerful, and one of very few Special Fighting moves. It helps many Special Attackers get good Normal/Dark type coverage, (Alakazam and Gengar come to mind,) but tends to fall short to Aura Sphere, with better accuracy and more PP, just with a lower Base Power.
With it's poor accuracy, Focus Blast just falls too short for me. It could work on Doubles paired with a Machamp, (which might actually work pretty well,) but it's negatives catch up to it's positives. If you ran Life Orb instead of Electric Seed, Grass Knot/Surf would work. In that case you would probably change Rising Voltage to Thunderbolt, and you would end up with ~120 Base Power for both TBolt and Surf.
If you don't want to change from Electric Seed to Life Orb, then I would definitely go with Surf. Good Base Power, coverage against a lot of types, and good PP are not all things that Grass Knot and Focus Blast can claim to have.
Hope I could help!