Basically Black Knight throws his Ebony Blade and it went at similar speeds to Akron’s lightning bolt
Now Akron's lightning bolt is lightning, it can have the power of 20 tons of tnt and has been commented by Storm as lightning
Alright so the Ebony Blade weights 50 pounds
50 lb = 22.6796 kg = M
Speed of lightning 440,000 meters a second = V
Ek=0.5*M*V^2
0.5*22.6796*440,000^2
2195385280000 joules
Final result: 524.7096749521988 Tons of TNT or Multi-City Block level