How do you get blue orb on soul silver not heart gold?

You can't get the Blue Orb in SoulSilver. It's exclusive to HeartGold.

The Blue Orb is essential for obtaining the legendary Pokemon Groudon in HeartGold. You receive it from Professor Oak after capturing Raikou, Entei, and Suicune.

There are no equivalent items or methods to obtain Groudon in SoulSilver.