It depends on the nature of the concentration
The sequester spell says that the affected target "falls into a state of suspended animation", and that "time ceases to flow" for it. Basically you are frozen, you can't perceive your surroundings or take actions in this state.
The rules describe concentration as a conscious activity the caster performs. Apparently, if a caster is frozen in time, he/she can not do such an activity — can't think, can't perceive their surroundings, can't concentrate on something. But, strictly saying, is unclear if concentration would or would not work in this state — the rules are silent about this, making this a subject of a DM fiat.
Any reasoning would be valid:
- The target falls into a state of suspended animation, one second lasts forever for it, it goes into a frozen-in-time state, so it can not perform any conscious activity in real time, including maintaining concentration (a simulationist's approach)
- The Sequester spell details don't say the target is incapacitated, therefore, it can maintain concentration (a strict rules-as-written approach)
However, Bless still lasts 1 minute, no more
Unlike Time Stop, Sequester only stops time for one person. If this person cast the Bless spell, its effect still last no more than the spell duration is (1 minute in this case).
The last but not least. When a DM is making adjudications, they should think about players having fun, not the rules:
Always follow this golden rule when you DM for a group: Make decisions and adjudications that enhance the fun of the adventure when possible
If you find it ingenious enough for an evil NPC wizard in your campaign, just let him/her do that (by inventing a custom spell, I guess). If a PC wants the same, though — that probably could break your game (but this is another question).