Sonic Heroes was released in 2003, and Blaze and Silver were introduced much later in the Sonic franchise.
* Blaze first appeared in Sonic Rush (2005) for the Nintendo DS.
* Silver made his debut in Sonic the Hedgehog (2006) for the PlayStation 3 and Xbox 360.
Therefore, they were not available as playable characters in Sonic Heroes.
However, you can play as these characters in other Sonic games, like:
* Sonic Rush (Blaze)
* Sonic the Hedgehog (2006) (Silver)
* Sonic Riders (Blaze and Silver)
* Sonic Rivals (Blaze)
* Sonic Generations (Blaze and Silver)
* Sonic Frontiers (Blaze)
* Sonic Colors: Ultimate (Silver)
* And many more!
If you're looking for a Sonic game with Blaze and Silver, you'll need to check out games released after Sonic Heroes.