Spells Do What They Say.
Jeremy Crawford has said numerous times, "Spells do exactly what they say." That if they require a roll, the spell will say so.
Design and History
Older versions of DnD had spell failure. 5e removed it because you're already spending an extremely limited resource (spell slots) to cast it. There are ways to force spells to fail, but there isn't just a random chance the spell does nothing.
When There are Rolls
When casting a battle spell there is typically an attack roll or saving throw to give the opponent an attempt to avoid or decrease the impact of the spell (there are some exceptions like Magic Missle). Missing or the opponent dodging or saving vs damage isn't spell failure; though if there is a single target the effect is basically the same as if there were.
When casting healing or buff spells, the spell typically just works in 5e. The rolls involved (if any) typically tell you how much health/how effective the effect.
So, When is Bless is Successful?
Bless just works when you have a focus or material components, speak the incantation and do the hand motions with a free hand. It is always successful. Once cast it will continue in operation until
- 1 minute passes,
- you cast another concentration spell,
- you use the Ready Action to ready a spell,
- you use a feat or feature that requires you to concentrate as if casting a concentration spell, or
- you take damage and fail the Constitution Save to maintain concentration.
Yes, it is a powerful spell, but it using two resources: concentration and a spell slot.