Yes, it is potentially dangerous. Do not take a beta blocker with Phenethylamines! Some of the Phens are actually psychedelic amphetamines.
It's only safe to take beta blockers with tryptamines (except for AMT and 5-MeO-AMT). I've done it especially with 4-HO-MiPT and 4-HO-MET as they raise my blood pressure to an annoying point some times.
I've not used atenolol myself, only propranolol and bisoprolol. But I did some googling for you and atenolol has a half-life of 6-7 hours, however the actual effects last up to 24 hours. So I'd say you should wait at least 24 hours before taking 25I-NBOMe.
Do you use atenolol daily? If so there's another problem. Discontinuing the use of beta blockers if used for a prolonged time should be done by taper, otherwise you can get very uncomfortable heart palpitations. You can't just stop 50 mg atenolol cold turkey if you've used it for a long time.
Also remeber that 25I-NBOMe raises you blood pressure, if you have BP issues I strongly urge you to find a psychedelic that doesn't affect blood pressure as much. Those can be found mostly in the Tryptamine class (you can anyway take atenolol with them).