Martin Chapman, 35, was caught on road cameras using the light on 38 separate occasions, including during one "dangerous and reckless manoeuvre".
Chapman told police he had joined the blood service and had intended to use the blue light "to do urgent blood runs between hospitals".
He will be sentenced on 10 February.
Chapman, of Connaught Street, The Mounts, Northampton, pleaded guilty to five counts of using a vehicle fitted with a "blue warning beacon" without permission.
Footage in which he "narrowly missed" a cyclist as he drove through the wrong side of a busy junction was played to magistrates.
This happened on 6 July on Cliftonville Road in Northampton and was described by the arresting officer as a "dangerous and reckless manoeuvre".
In giving evidence, Chapman admitted he had been driving the car but argued that he had not received notice of prosecution in time.
The court also heard that, after his arrest, he told police he had applied to join the blood service and that the light would have been used to do urgent runs between hospitals. However, he later admitted that he had no contracts for such work.
Chapman was found guilty of one count of dangerous driving, three counts of ignoring a red light and road markings and five charges of using a car without correct insurance.
He told the court he was "remorseful" for what he had done.
Magistrate chair, Jackie Shore, said the dangerous driving offence was "so severe" that sentencing would have to take place at Northampton Crown Court.
A man who attached a flashing blue light to his Vauxhall Astra to get through Northampton traffic has been convicted of dangerous driving.