The beholder emits an Antimagic Cone:
The beholder's central eye creates an area of antimagic, as in the antimagic field spell, in a 150-foot cone. At the start of each of its turns, the beholder decides which way the cone faces and whether the cone is active. The area works against the beholder's own eye rays.
According to the Basic Rule, Ch. 10, Cone:
A cone extends in a direction you choose from its point of origin. A cone's width at a given point along its length is equal to that point's distance from the point of origin. A cone's area of effect specifies its maximum length.
A cone's point of origin is not included in the cone's area of effect, unless you decide otherwise.
Can a beholder chose to include itself in its Antimagic Cone? Would that make it effectively immune to harmful spells like Blindness/Deafness?