As the question states, if a creature has blindsight, do they have disadvantage on a target that is within the range of their blindsight and not otherwise hidden/covered?
The wording of the invisible condition suggests that attacks against an invisible creature have disadvantage AND the creature can't be seen without magic/special sense not BECAUSE the creature can't be seen.
An invisible creature is impossible to see without the aid of magic or a special sense. For the purposes of hiding, the creature is heavily obscured. The creature's location can be detected by any noise it makes or any tracks it leaves.
Attack rolls against the creature have disadvantage, and the creature's attack rolls have advantage.