Police said the body of the suspected woman bomber was believed to have been among others found on the roadside in Maiduguri, the capital of northeast Borno state.
It was not immediately clear if she was counted in the official death toll.
"About eight persons lost their lives, 15 injured persons were evacuated to specialist and teaching hospitals," the National Emergency Management Agency (NEMA) said in a statement.
A small crowd gathered following the explosion to see the charred wreckage of the yellow and green taxi cab as emergency workers moved human remains into black body bags nearby.
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"I am disheartened by the news of the bomb blast in Maiduguri. My condolences to the families of the victims, and the Borno State Government," said Nigerian President Muhammadu Buhari in a statement on Twitter.
The bomb was detonated on a road leading to towns close to the Cameroon border that until recently were inaccessible because of Boko Haram attacks.
Boko Haram has escalated its attacks on Nigeria's neighbours in recent months.
In Nigeria, a sustained counter-offensive has seen the military retake swathes of territory from the insurgents, leading to a relative lull in attacks.
But Boko Haram still poses a security threat to civilians and has attacked vehicle convoys in recent months.
Last month, six people were killed in an ambush on a commercial convoy under military escort in the Sanda district of Borno state, while earlier this year a UN humanitarian convoy travelling by road from Bama to Maiduguri was ambushed.
"We have seen a lull in Boko Haram's activity in Nigeria's northeast... But we do generally see an uptick of activity after the rainy season ends."
Boko Haram's seven-year insurgency has left at least 20,000 people dead in Nigeria and border areas of neighbouring Niger, Chad and Cameroon, and made more than 2.6 million homeless.