The exchange rate 1BTCD=50KMD will be the same for all the period of the swap (1 year) ?
Basically yes, but the exchange rate will decrease slightly every day to account the 5% APR bitcoindark has. With this we ensure there won't be more BTCD than there are KMD reserved for the swap.
So apparently there isnt any difference partecipating the ICO (as i done) and buying now BTCD and wait the swap?
It doesn't seem so, at least for the time being. Maybe in some days/weeks/months, depending on how the rate will decrease. Now, with the current rate of 1 BTCD = 50.42041779 KMD, it is more profitable/less risky to buy BTCD (if there's enough liquidity) and to swap it.