Even after it has been bloomed, dry gelatin is still not completely rehydrated. When you mix it into the liquid, the gelatin is still visible. The liquid must be heated in order to thoroughly dissolve the gelatin. Stir the liquid so the gelatin does not fall to the bottom of the pan and scorch.
Protein gels are weakened by high heat or prolonged heating. Therefore, do not boil the gelatinized liquid nor hold it at a high temperature for a long time. Heat the liquid to 180°F (82°C) and hold it there until the gelatin is no longer visible.