We're guessing you already know that a story with major blackouts isn't a good thing because you know all about the difference between light and dark imagery from your English classes. Light = good; dark = not so good.
And, in fact, you would be right because the blackouts in ATM basically are a huge flashing sign telling you that this story won't be happy.
The blackouts aren't that simple because, if you're reading the story carefully, you might find yourself getting persuaded by Shukumar's optimism. To Shukumar, the blackouts signify honesty and intimacy with Shoba, both the verbal kind and the marital kind:
Something happened when the house was dark. They were able to talk to each other again. The third night after supper they'd sat together on the sofa, and once it was dark he began kissing her awkwardly on her forehead and her face, and though it was dark he closed his eyes, and knew that she did, too. The fourth night they walked carefully upstairs, to bed, feeling together for the final step with their feet before the landing, and making love with a desperation they had forgotten. (ATM 88)
You can see why Shukumar sees these blackouts as an opportunity for connection and why he's hopeful that their relationship might return to what it used to be.
And he's not totally off-base. The couple does seem to have some honest self-disclosure during these blackout sessions. Honest, maybe, to a fault, especially in Shoba's case:
On the third night, she told him that once after a lecture they'd attended, she let him speak to the chairman of his department without telling him that he had a dab of pâté on his chin....The fourth night, she said that she never liked the one poem he'd ever published in his life….He'd written the poem after meeting Shoba. She added that she found the poem sentimental. (ATM 87)
Notice how Shoba uses the blackout as an opportunity to spill all the negative stuff she never bothered to reveal before? That's an added tip to you that these blackouts really just mean negation, specifically, the negation of their relationship.
That's very clear at the end, when the lights are back on and Shoba decides to tell Shukumar she wants to separate from him. Shukumar, in turn, decides to be just as revealing and tells her the one thing she doesn't want to know: the sex of their stillborn baby (it was a boy). In response, she turns off the lights and they "[weep] together for the things they now [know]" (ATM 104).
Why does she turn off the lights herself? Think of the color of grief and mourning—black. The final way you can read the blackout is to link it straight back to source of the couple's problems: the death of their child.