The errata doesn't change the rule, it only clarifies it.
Normal darkness creates a heavily obscured area, which you can see out of. For example, in a darkened room a creature with normal vision can see a creature standing in a well lit adjoining room with no intervening obstructions.
The original wording:
A heavily obscured area–such as darkness, opaque fog, or dense foliage–blocks vision entirely. A creature effectively suffers from the blinded condition (see appendix PH-A) when trying to see something in that area.
Vision and Light (p. 183). A heavily obscured area doesn’t blind you, but you are effectively blinded when you try to see something obscured by it.
Beginning from disregarding the Darkness spell entirely, both wordings are essentially saying the same thing. If you are trying to see something in a heavily obscured area, you can't.
It's entirely possible an area could be obscured in both directions, a thick tapestry or dense hedge or fog could prevent two creatures on opposite sides from seeing each other, effectively making the heavily obscured area everything on the other side of the obstruction. It doesn't preclude an obscured area being one way only, it certainly should not be the default presumption.
Regarding the Darkness spell, it's a case of specific beats general.
Magical darkness spreads from a point you choose within range to fill a 15-foot-radius sphere for the duration. The darkness spreads around corners. A creature with darkvision can’t see through this darkness, and nonmagical light can’t illuminate it.
Let's look at the section
A creature with darkvision can’t see through this darkness,
Darkvision as described in PHB 183, specifies:
[...] Within a specified range, a creature with darkvision can see in dim light as if it were bright light and in darkness as if it were dim light, [...]
This is a specific exception to the general rules.
Darkness creates a heavily obscured area. [...]
A heavily obscured area - such as darkness, opaque fog, or dense foliage - blocks vision entirely.
The wording in the spell Darkness is an even more specific rule overriding the exception of the darkvision. Nothing in the spell or rules preclude the possibility of seeing out of magical darkness.
Furthermore, the darkness can in fact be illuminated. Even by something as simple as Faerie Fire.
If any of this spell’s area overlaps with an area of light created by a spell of 2nd level or lower, the spell that created the light is dispelled.
The intent appears to be that Darkness will snuff out Light and first and second level spells that create light, (and in turn be snuffed out by Daylight,) but consider
Casting a Spell at a Higher Level
When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts Magic Missile using one of her 2nd-level slots, that Magic Missile is 2nd level.
Therefore objects and creatures affected by the Faerie Fire spell cast using a 3rd-level slot can illuminate, without dispelling, the magical darkness created by the Darkness spell.
The interpretation that magical darkness blocks light from passing through, rather than just prevents illumination, brings up other questions as well.
Completely covering the source of the darkness with an opaque object, such as a bowl or a helm, blocks the darkness.
If the magical darkness were, in fact, opaque, would two spheres of magical darkness completely covering each other's points of origin cancel each other out?
Consider also the magical darkness of the Hallow spell, that does not block darkvision and certain creatures can ignore. If this were opaque darkness, how could a creature making a save or excluded from the effect ignore it?
Consider the 20-foot-radius sphere of blackness created by the 3rd level spell Hunger of Hadar
No light, magical or otherwise, can illuminate the area, and creatures fully within the area are blinded.
If a similar effect were intended for the 2nd-level Darkness spell, why wouldn't such language be included in the wording of the spell?
In conclusion, without making "logical leaps" we can see magical darkness is not opaque.
It does however block vision entirely when a creature (without a more specific exception,) tries to see into the heavily obscured area it creates.