the "silver" spokes can either be Stainless steel, or Galvanized steel... the stainless spokes are preferred due to their resistance to rust, and a bit of extra "strength" from the elasticity of most SS alloys compared to galvanized steel.
the "black" spokes are usually painted Galvanized spokes. a couple brands offer painted SS spokes.
Galvanizing lends some brittleness to the base metal. Zinc is Brittle.
the difference in strength is not enough to be a real concern.

i've built up a couple sets of wheels using half black, half shiny spokes... SS in the rear wheel's "pull" direction, black in the other.
didn't like the look, personally.
i use black spokes with brass/chrome nips, and black rims... that looks good, adds some bling to the wheels.
all silver spokes and nips with black hub and rims too.
silver hub, black spokes, chrome nips, black or silver rims...

i have not yet tried a "one side black, one side silver" build... maybe next time?