HarryM

The production of coffee bloom is an essential element of fresh ground coffee when infused with 92c water.

When you pour/infuse 92c water over freshly ground coffee, you'll begin to see the ground coffee come alive especially when you stir the grounds around in the water a little; you should notice a lovely fizz effect on top like a volcano erupting which is created by Co2 from the beans and it's this which gives the 'bloom' of the coffee. If you see little to none, you know that the coffee is spent/shot, and isn't suitable for drinking!

In short, the fresher the coffee, the more bloom you will see. After a week or so (max 3 weeks), you will see far less to no blooming.

Dark or Light Bean Roasts
The lighter the bean roast, the more thicker a bloom/head you'll get!