I heard a Fly buzz-when I died
The speaker describes what was in her mind and her feelings toward the surroundings at the time when she is going to die. She lies on the bed and describes her feelings.
In the first stanza, the speaker heard a fly buzz when she died. I think because the speaker is going to die, she gradually loses the ability of seeing. She can't see clearly, but she still can hear some sound. The room is very quiet and she can hear a fly buzzing. Because she only has the ability of hearing, she can hear some subtle sound which general people usually don't pay attention to. "Between the Heaves of Storm-" I think it represents that the speaker's situation. She is between living and dead.
"The Eyes around- had wrung them dry-" it means that the speaker's family and relatives are by her side, and that they are so sad that their tears have been all shed. They can't cry with tears. "And Breaths were gathering firm" It is so quiet in the room that the speaker can feel that the people around her breathe slowly and that they try to be strong. They try not to show sadness so that the dead can leave this world without worrying.
"For the last Onset- when the King
Be witnessed- in the Room-"
I guess the king is God, because when someone died, only God could witness the whole process; so the speaker feels she is witnessed. God is by her side and will see the process of her death. Besides, she thinks dead is the last onset, because after dead, you don't have to face any difficulties or frustrations. You don't have to suffer pains afterwards.
In the third stanza, the speaker singed away what portion of her is assignable. She had already prepared for dead and decided to give up something concrete, such as money, house or something like that. She only can sign away her possessions, and leave those to her family; but she can't give up her spirits and her mind.
"- and then it was
There interposed a Fly-
With Blue- uncertain stumbling Buzz- "
The fly interposed, and the speaker again hears its buzz. Because the eyes of a fly are blue and the speaker can't see clearly, she only see the blue color. Besides, she is also going to lose the ability of hearing, so she hears uncertain stumbling buzz and she can't hear very clearly, only on-and-off sound.
"And then the Windows failed-" We often say that eyes are the windows of our souls. When the windows failed, the speaker can't see at all and she is dead. "- and then I could not see to see-" There speaker has been dead and she can't see anything through her eyes.
I don't know why Dickensen can write so delicately about death. The words she wrote seem that she had experienced death. I think she must be a sensitive writer.