It’s a good question. I think the difference between being blissful and being happy would become clearer not by defining them separately but by comparing them. Bliss is a great deal stronger than happiness. You can be happy because it’s not raining today, or because you found your glasses. But bliss is reserved for a state of complete, perfect happiness, such as what you may experience by finding your soulmate or having a transcendent spiritual experience. Like

Srimate Ramnujaya Namaha,

Namaskaram Swami,

As per reading texts my inference is Happiness is within 3 gunas Satvam, Rajas and Tamas. Bliss is supposed to be beyond these 3 modes. When soul is in its original nature, sat-cit-ananda roopam, it may experience bliss/ananda.

Another rough example to realize in current context (happiness is when we get something when someone helps, bliss is when we are being able to act as an instrument of God and help others).

Adiyen