This thread of tweets by the game's designers about Eldritch Blast has a lot of interesting information that is relevant to this question, if not directly answering it (since they also say we can imagine it how we want):
Designers: It's force and damage can't target objects, only creatures
@jimcullenaus: Sage Advice says EldritchBlast&similar can't cast on objects at all. @JeremyECrawford or @mikemearls, how do you justify this with flavour?
@mikemearls: [Eldritch Blast description] specifies creature - always assumed it disrupted life force via force energy
@JeremyECrawford: The definition of force damage, not of force itself, is "pure magical energy focused into a damaging form" (PH, 196)
It isn't a "projectile" because it's not shot from a weapon
@JeremyECrawford: Some spells are drawn to or harm only the life force of creatures. You're not shooting projectiles. #DnD
@JeremyECrawford: Traditionally, a projectile is a thing shot from or thrown as a weapon. This is common parlance, not a rule.
- However you imagine your EB looking and sounding, it makes sense to account for the fact that it can ONLY affect the life force of a creature. It can't hit objects or anyone but the target creature. Does your idea of a "Crackling beam streaking" toward a target fit this description?
- It's not a "projectile", but instead a "beam of energy", but JeremyECrawford's definition of a projectile is extremely limited and wouldn't describe any version of EB, so that doesn't prove much.
- It could still be a projectile by my personal definition, which would surely include the energy blasts that come out of Star Wars blasters, whether it was generated by a weapon or the mind of a magical caster.
- My personal definition of a "projectile" would exclude a laser or "beam" though, as to me both of those make a bridge to the target instantaneously, and because they don't "travel", the "projectile" label feels wrong. I'm not sure if this is part of what JeremyECrawford was thinking when saying that EB is a "beam" and not a "projectile", but his definition of a projectile has nothing to do with this question of laser v. bolt.
Laser? Blaster? Arcing bolt?
A beam of crackling energy streaks toward a creature
To me all this leaves open the important question of whether the "beam" is a continuous line between the caster and target (like a laser), whether it's a discrete blob of energy that goes slow enough to watch it travel to the target (like a Star Wars blaster), or a zig-zagging energy effect like a lightning bolt.
One of the main questions to me is it's "speed". If it's not a laser that totally fills the space between the caster and target, then does it still travel fast enough to be instantaneous, which would make it effectively equivalent to a laser in terms of visible display (a solid glowing line), or slow like a Star Wars blast that you can see moving (i.e. slower than a bullet, let alone the speed of light).
The word "streaking" really doesn't sound like a laser beam to me, but it doesn't sound like a Star Wars blast either. Is it just a description of the fast travel speed of the effect?
To me a lightning bolt seems like the effect least well described by "energy beam", and I'm inclined to ignore this option because of it, but on the other hand, the word "streaking" implies some kind of zig-zagging reminiscent of lightning-style effects, putting it back into the competition.
If I'm honest, I feel like the "A beam of crackling energy streaks toward a creature" description here is essentially incoherent, and if anything, it should be read as a series of "or" possibilities, rather than a cumulative "and" description:
A crackling beam or bolt or streak of energy
To me, the description leaves all this open, and it's easier to make sense of it as a description of various phenomena than of a single sensory experience.
Most important: The "streaking energy beam" makes noise and is visible
Ultimately I think most of these questions don't matter to the game, since based on what the designers have said, it hits an individual directly in their "life force", so regardless of how it looks, the visual display is a kind of sensory epiphenomenon, rather than an important part of the spell. A ghostly aparition that represents the spell, rather than a physical source of damage.
Functionally the damage could be delivered in a silent and invisible spell where the target just suffers damage internally, but the flavor makes it clear it that EB makes a noise and is visible, which is important for balancing the spell and giving enemies a chance to know what the hell is killing them.
While its tempting to flavor your EB as "made only of darkness, a shadow that slithers to the target", this is probably a little too convenient. An EB flavor should account for the "crackling", "energy", and "beam" aspects, and deal with the fact that it makes it easy to identify the source of the spell and its magical nature.