Friends of the family are now raising money to support Blackman, whose benefits have been stopped following Fenton's death on Friday, July 28.
Blackman's mother Rebecca Watson, 53, said his death was a "mystery" and the family man was "happier than ever" and had "everything to live for".
A woman in her 40s was left in a critical condition after a fall from the same cliffs in Cornwall on August 1 which also crumbled on to families below on July 30.
Speaking on behalf of her distraught daughter, Watson said: "He got diagnosed eight or nine months ago and it was one of the most aggressive forms of cancer and in his heart, lungs and lymph nodes.
"He had six months of chemo and we thought we were going to lose him then. It was a miracle he got through it.
"The cancer brought us all closer together. We were going up and down from Derriford Hospital. The tumours were so big. It was so much.
"We never thought he would come out the other side. But he did. They said that the tumours had all shrunk.
"They couldn't say he was in remission but he had a scan and they had all gone.
"We were celebrating. We thought he would never get through it but he responded to the treatment. It was unbelievable.
"He got the all clear three weeks ago and now he's dead.
"They were about to get married and now the songs they picked for their wedding will be played at the funeral."
She added: "My friend started the fundraising page to help Carly. We are just so numb."
Fenton was diagnosed with B-cell lymphoma - blood cell tumours - around nine months ago.
Doctors discovered tumours on his heart, lungs and lymph nodes, and his family feared he would "never come out the other side", Watson said.
But with the support of his family, he endured months of super-strong chemotherapy which miraculously shrank the tumours.
Just three weeks before his death a scan revealed the tumours were no longer visible, and he was given the all clear, according his family.
After eight-and-a-half years together Adam and Carly planned to get married with a small ceremony later this year.
The proud father was still too poorly to work, but gathered the strength for his first night at the pub with mates on July 28.
"In a way [he was out to celebrate his all clear]," said Watson.
"It was just a couple of drinks. It was the first time he had been out in ages."
Police are still trying to piece together what happened, but it is understood he disappeared and friends thought he had simply gone home, she said.
Blackman went to bed after settling the kids, and woke up wondering where Fenton was - until police knocked on the door to say his body had been found.
A dog walker found him at 6.30am and his death was now being treated as unexplained.
"It's a mystery what happened," said Rebecca.
It is so early days but there is only a metal pole - a barrier - and if you walked up to the bush [at the edge] you would never think that the cliff edge was there.
"He knows Newquay. It is all a mystery to us. I don't think he did it on purpose. He had absolutely no reason.
"He had his children, he had his family, he had the all clear, and he had everything to live for.
"He was so happy. I can't imagine him doing that. He was the most lovely and most loving and happy man. He was the most friendly person you would wish to meet.
"It doesn't make sense at all. Telling the youngest girls was heartbreaking.
"They've been going to and fro from hospital helping their daddy fight cancer and now this."
Watson wants the authorities to make the cliff safer, and speculated he entered the bushes not realising there was a sheer drop at the other side.
Watson claims when the family informed HMRC of his death they mistakenly stopped ALL Blackman's benefits, leaving her struggling financially while she is grieving.
A family friend has set up an online fund to support the family, and while the page said will be used for a holiday, Watson said it is simply needed for "day to day costs".
So far friends have raised £1035 ($1833).
"He was on ESA because the doctor said he needed to rest for three to six months,' said Watson.
"I'm sorting out her paperwork and I phoned up to inform them of his death as I didn't want Carly getting any trouble.
"But they have stopped everything - even the child tax credits.
"I thought they would have understood she was there by herself.
"They have abandoned her when she needs help the most. She's not even entitled to a widow's pension as they weren't married. I can't understand.
"Any help would be lovely. We are all so numb."
A spokesman for Devon and Cornwall Police said: "We were called to the Towan Beach at 6.36 this morning after a male was found on the beach by a member of the public.
"The ambulance service attended and hew as pronounced dead at the scene.
"At the moment we are treating the death as unexplained."
