The blackout didn't keep people honest, quite the reverse; Joshua Levine describes the period as a "Golden Age of Crime" in a chapter title of The Secret History of the Blitz and there is both statistical and anecdotal evidence of an increase in crime during the war. Juliet Gardiner, for example, gives the figure of 303,771 indictable offences in 1939 in England and Wales, rising to 478,394 in 1945. As Marc Wiggam notes in his thesis The Blackout in Britain and Germany during the Second World War, though, some care has to be taken with crime statistics as "... many crimes may have gone unrecorded, collection of data may not have been uniform across towns and cities, figures may have been massaged for political purposes, and detection and conviction rates may simply indicate more attention paid to those crimes than had previously been the case".

Part of the increase can be attributed to the creation of new regulations - lighting restrictions to enforce the blackout itself, rationing offences, "defeatist talk" and such. Wartime regulations could sometimes indirectly assist the police; the internment of 'enemy aliens' included Italians from June 1940, and led to members of the Sabini gang (as fictionally portrayed in Peaky Blinders) being detained even after other internees were released, though their absence led to other gangs fighting to fill the vacuum. In the main, though, career criminals took advantage of the cover of darkness and dislocation of wartime. Gangs of safe-crackers were estimated to have stolen £20,000 across London by posing as emergency workers, falling bombs covering the sound of gelignite explosions. Opportunists also seized their chances, looting was common - Levine gives the example of Jack Miller of the King's Own Royal Regiment, sent to Coventry to assist with rescue operations in November 1940, who observed his comrades pilfering watches, jewellery and cash from shops between their duties. As a judge in Sheffield put it: "The task of guarding shattered houses from prowling thieves, especially during the blackout, is obviously beyond the capacity of any police force", and with regular police leaving to join the armed forces and additional duties such as assisting recue squads the police were particularly stretched.

Travelling during the blackout was dangerous both due to accidents - road deaths in London increased during the first two years of the war despite reduced traffic - and criminal activity. As Wiggam puts it "For women in particular, the potential for inappropriate advances, stalking and sexual violence was far greater under the cover of darkness". Observers noted that men and women tended to cluster separately at e.g. train stations, Glasgow police advised that female ARP wardens should be escorted home. The darkness also offered cover for other activities, with an increase in prosecutions for homosexuality (criminal at the time) and prostitution (street prostitutes in Manchester were nicknamed 'fairies' for the white coats they wore to show up in the blackout). West End 'Bottle Party' clubs thrived, circumventing licensing laws by posing as invitation-only clubs where patrons (theoretically) brought their own drinks, drawing the attention of the Vice Squad.

Despite the increase in offences Wiggam notes the "consensus amongst Police Chiefs in Britain shows a satisfaction with the level of crime throughout the war" as the greater opportunities for criminals and difficulties in policing during wartime had been anticipated. One area that did cause particular concern was juvenile crime, with families disrupted by evacuation or absent parents called up to the armed forces or essential war work. Glasgow's Police Chief reported increases in "Theft, Housebreaking [...] Malicious Mischief and contraventions of the Glasgow Police Acts (Stone Throwing, Hanging on Rear of Vehicles, etc.)"; youth punishments rose from 1,919 in 1939 to 2,809 in 1940 with particularly dramatic increases in fines (93 to 396) and whippings (14 to 111). Manchester's Police Chief noted a similar increase in juvenile crime. These were the more extreme cases reflecting wider issues in family life, education, and truancy brought by the war that fed into post-war social reforms.