Yes. For a given definition of yes.

You ask "can it be seen normally" but normally would generally infer being seen with a creature's normal eyesight. Obvious if it is heavily obscured that isn't going to be the case:

A heavily obscured area—such as Darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the Blinded condition (see Conditions ) when trying to see something in that area.

However, blindsight allows a creature to "see" even when their eyesight (if any) would normally be blocked:

A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense.

That being said. Blindsight only works within a certain range (specified in the stat block for the creature). If the creature is within that range, and their blindsight hasn't been blocked (echolocation, for example, can be blocked by deafness/silence effects).

As such, a creature with blindsight won't get disadvantage on attacks if they can "see" their target using that ability.

Yes. For a given definition of yes.

You ask "can it be seen normally" but normally would generally infer being seen with a creature's normal eyesight. Obviously if it is heavily obscured that isn't going to be the case:

A heavily obscured area—such as Darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the Blinded condition (see Conditions ) when trying to see something in that area.

However, blindsight allows a creature to "see" even when their eyesight (if any) would normally be blocked:

A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense.

That being said. Blindsight only works within a certain range (specified in the stat block for the creature). If the creature is within that range, and their blindsight hasn't been blocked (echolocation, for example, can be blocked by deafness/silence effects).

As such, a creature with blindsight won't get disadvantage on attacks if they can "see" their target using that ability.