Yes, because eldritch blast consists of multiple attacks.
The rules on making an attack are as follows:
Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.
Choose a target. Pick a target within your attack's range: a creature, an object, or a location.
Determine modifiers. The DM determines whether the target has cover and whether you have advantage or disadvantage against the target. In addition, spells, special abilities, and other effects can apply penalties or bonuses to your attack roll.
Resolve the attack. You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.
If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack.
As you can see, if something involves an attack roll, it is an attack. You choose a target for the attack, determine relevant modifiers to the attack roll, and resolve the attack (by making the attack roll, seeing if it hits, and rolling damage and resolving any other effects).
The eldritch blast cantrip consists of at least one attack roll, of course - but at level 5 and up, it consists of multiple attack rolls:
A beam of crackling energy streaks toward a creature within range. Make a ranged spell attack against the target. On a hit, the target takes 1d10 force damage.
The spell creates more than one beam when you reach higher levels: two beams at 5th level, three beams at 11th level, and four beams at 17th level. You can direct the beams at the same target or at different ones. Make a separate attack roll for each beam.
Obviously, at level 1-4, it is clearly an attack. Now the question is: at level 5 and up, is it considered one attack, or multiple attacks?
By default, anything that consists of multiple attack rolls consists of multiple attacks. This is clear by looking at the rules for making an attack quoted above; it is a sequence of targeting something, determining your modifiers, and making and resolving the attack roll.
Unless stated otherwise, anything that involves multiple attack rolls - including eldritch blast for higher-level characters - generally has you go through this process, step by step, for each attack. Rules designer Jeremy Crawford seems to support this claim in his response to a related tweet in an unofficial tweet from June 2015:
Eldritch Blast: are the attacks resolved in parallel or sequence? Do you have to pick all the targets first before rolling?
Multiple attacks on the same turn aren't simultaneous, unless a feature or spell says otherwise.
Clearly, each beam of eldritch blast goes through the steps of "making an attack" from the beginning, one after the other; you choose a target for each beam, determine modifiers (though this will almost certainly be the same for every beam unless something grants advantage/disadvantage on a single attack), and resolve each attack separately.
Crawford also unofficially addressed this claim in his response to another tweet asking about exactly this back in September 2014:
Was it intended for Eldritch Blast to be considered one attack with multiple attack rolls & thus to activate Hax only once?
Hex triggers each time you hit the hexed target, so eldritch blast can trigger it on each hit.
The hex spell (PHB, p. 251) says:
Until the spell ends, you deal an extra 1d6 necrotic damage to the target whenever you hit it with an attack.
As Crawford states, hex causes each attack of yours that hits to deal extra damage. At higher levels, eldritch blast consists of multiple beams, each with a separate attack roll. If, as Crawford states, eldritch blast triggers the extra damage of hex on each hit, then by necessity, each beam of eldritch blast must be a separate attack (as part of the same spell).
Finally, we can compare eldritch blast to a different feature that is considered "one attack" despite having multiple attack rolls, according to the Sage Advice Compendium: the Hunter ranger's Whirlwind Attack feature.
Can a ranger move between the attack rolls of the Whirlwind Attack feature?
No. Whirlwind Attack is unusual, in that it’s a single attack with multiple attack rolls. In most other instances, an attack has one attack roll. The rule on moving between attacks (PH, 190) lets you move between weapon attacks, not between the attack rolls of an exceptional feature like Whirlwind Attack.
So if Whirlwind Attack consists of multiple attack rolls, why is it considered "a single attack" - and what makes it exceptional or different to other things with multiple attack rolls?
To answer that, we need to look at the description of the feature. The Hunter ranger's Whirlwind Attack option for its Multiattack feature says:
You can use your action to make a melee attack against any number of creatures within 5 feet of you, with a separate attack roll for each target.
Note that the other option for the Multiattack feature, Volley, seems to be worded similarly:
You can use your action to make a ranged attack against any number of creatures within 10 feet of a point you can see within your weapon’s range. You must have ammunition for each target, as normal, and you make a separate attack roll for each target.
Whirlwind Attack, like eldritch blast, consists of a separate attack roll for each target. However, the key difference seems to be that the feature itself says you make "a melee attack" (singular) against multiple creatures, rather than "melee attacks" (plural).
The same holds true for Volley, except it says "a ranged attack" (still singular); though not mentioned in the SAC ruling, Crawford confirmed that it's also considered a single attack in this unofficial tweet from September 2014.
It's definitely a weird bit of phrasing to hinge upon, but it's the only thing exceptional about the feature. The distinction can't hinge on what's left unstated; there are no secret rules. Therefore, the thing marking Whirlwind Attack and Volley as "a single attack" must be the fact that they tell you to make "a [melee/ranged] attack", despite targeting multiple creatures.
Otherwise, every single spell that consisted of multiple attack rolls - such as scorching ray, or chaos bolt if the energy leaps, or any single-target attack as part of a spell that then has the sorcerer's Twinned Spell metamagic option used on it - could also be considered "a single attack"... which would make Whirlwind Attack (and Volley) very unexceptional.
