C
Note: This answer does not go into the base power calculation of hidden power, as gen 6 and above normalized hidden power's base power to 60.
The type of hidden power is determined by the least significant bit (or odd/even-ness) of each of your IVs by the formula
let a, b, c, d, e, f = HP, Atk, Def, SpA, SpD, Spd IV mod 2
type = Floor( (a+2*b+4*c+8*d+16*e+32*f) * 15 / 64 )
where the resulting number type is an integer from 0-15, and the type number is mapped to a hidden power type. Since we are looking at hidden power dark, we want a type of 15, which is only possible when a,b,c,d,e,f are all odd, or the pokemon's HP, Attack, Defense, Special Attack, Special Defense, Speed IVs are all odd.
Given that you are chaining pokemon, 4/6 of your IVs are 31, or odd. This leaves 2 IVs that are evenly distributed over [0-31]. Since you know the pokemon's hidden power type is dark, the non-perfect IVs are (1,3,5,7.... 31). There is a 1/16 chance for each of the two IVs to be 31, or 1/16*1/16 = 1/256 chance that both are 31 (0.390625%) and you have a perfect pokemon.