The rare animals were among 14 being taken to the Tsavo East reserve in southern Kenya from their homes in Nairobi and Lake Nakuru national parks.
Officials believe the rhinos may have died from salt poisoning after drinking salty water in their new environment.
Dr Paula Kahumbu, a Kenyan wildlife conservationist and CEO of the WildlifeDirect, said it was astonishing that such catastrophic mistakes could be made, in a country so used to caring for its rhinos.
She said: “It’s a disaster – a disaster not just for Kenya, but for the world.
“These animals have been protected for decades by rangers.
“So for them to die like this is incredibly alarming.
“And I’m very surprised we haven’t been told yet what happened.”
Najib Balala, Kenya’s tourism and wildlife minister, ordered the Kenya Wildlife Service (KWS) to “immediately suspend the ongoing translocation of black rhinos following the death of eight of them”.
Mr Balala said that “preliminary investigations” suggested the rhinos may have died of “salt poisoning” after drinking different water in their new environment.
He said the animals likely became dehydrated and drank more salty water.
A full report is due to be produced next week.
A KWS spokesman said: “Disciplinary action will definitely be taken if the findings point towards negligence or un-professional conduct on the part of any KWS officers.”
Dr Kahumbu said it was vital to find out quickly what went wrong, to avoid future problems. She said animals have been known to die in the past from problems with food, water or the drugs for sedation during transit.
She said officials must take responsibility and explain what went wrong, and quickly.
She said: ”Rhinos have died, we have to say it openly when it happens, not a week later or a month later.
“Something must have gone wrong, and we want to know what it is.”
Save the Rhinos estimates there are fewer than 5,500 black rhinos in the world, all of them in Africa.
According to KWS figures, nine rhinos were killed in Kenya last year by poachers.
The last remaining male northern white rhino on the planet died in March in Kenya, leaving conservationists struggling to save that sub-species using in vitro fertilization.