(For those unaware, the SAC is list of rules clarifications written by the D&D game designers. As sometimes the Rules as Written are unclear, the SAC provides the developer’s Rules as Intended.)
Can a bonus action be used as an action or vice versa? For example, can a bard use a bonus action to grant a Bardic Inspiration die and an action to cast healing word?
No. Actions and bonus actions aren’t interchangeable. In the example, the bard could use Bardic Inspiration or healing word on a turn, not both.
As Eldritch Blast has the casting time of one action, it can not be cast as a bonus action. As stated above, actions and bonus actions aren’t interchangeable, the player could only cast Eldritch Blast with their action.
However, the Sage Advice Compendium also says this:
Is there a limit on the number of spells you can cast on your turn?
There’s no rule that says you can cast only X number of spells on your turn, but there are some practical limits. The main limiting factor is your action. Most spells require an action to cast, and unless you use a feature like the fighter’s Action Surge, you have only one action on your turn.
If you cast a spell, such as healing word, with a bonus action, you can cast another spell with your action, but that other spell must be a cantrip. Keep in mind that this particular limit is specific to spells that use a bonus action. For instance, if you cast a second spell using Action Surge, you aren’t limited to casting a cantrip with it.
Firstly, this further confirms that Eldritch Blast can not be used twice, though you can cast it and a spell with a casting time of a bonus action as Eldritch Blast is a cantrip. Secondly, this states that you could cast Eldritch Blast twice per turn if you had two actions per turn.
Finally, there is a section about casting reaction spells:
Can you cast a reaction spell on your turn?
You sure can! Here’s a common way for it to happen: Cornelius the wizard is casting fireball on his turn, and his foe casts counterspell on him. Cornelius also has counterspell prepared, so he uses his reaction to cast it and break his foe’s counterspell before it can stop fireball.
Now, whilst Eldritch Blast does not have any reactions attached to it in it’s description, the War Caster feat says:
when a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at that creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must only target that creature.
Theoretically, this means you could cast Eldritch Blast using your action then, if the creature moves, you could use it again, but only on the creature that moved. In this way, you could fire 4 Eldritch Blast beams at 5th level in a single turn (or 8 in one turn at 17th level), but half of them can only hit a single target, and only if their movement provokes an opportunity attack from you and you have the War Caster feat.
It's even possible that, by multi-classing and using feats, you could use Eldritch Blast even more times per turn. For example, combining Action Surge and the War Caster feat, you could: use an action to cast Eldritch Blast, Action Surge, use your second action to cast Eldritch Blast, opportunity attack is provoked, use your reaction to cast Eldritch Blast. A character who was a 5th-level Warlock and 2nd-level Fighter could fire 6 beams in a single turn this way; a 17th-level Warlock and 2nd-level Fighter could fire 12 beams a turn using this method.