The Association of Chief Police Officers (ACPO) issued the rules because Mr Blunkett plans to downgrade cannabis from Class B to Class C on January 29.
It means cannabis will move into the "least harmful" category under the Misuse of Drugs Act alongside anabolic steroids and some prescription anti-depressants.
Shadow home secretary Oliver Letwin said the proposals would cause confusion because they made cannabis "semi-legal".
But Mr Blunkett said the Government's policy on cannabis simply recognised what many police forces were already doing.
He said: "We believe that having a sensible consistency across the country, which distinguishes between what people are doing with less harmful drugs in their own homes from killer drugs in the community, is the right way forward."
Asked if he thought the guidelines sent out the wrong message to young people, he said: "The only wrong message that is being put out is those proclaiming that we've been legalising cannabis, which we have not, and those that have said that it isn't a long-term dangerous drug, which it is.
"What we have said is that it is not the killer and not the danger to the community that drugs like heroin and crack cocaine are."
The guidelines have not specified a maximum weight at which cannabis smokers can claim their stash was for personal use.
But they do define when officers should arrest people caught with the drug.
There are a number of situations in which people could be arrested, such as smoking cannabis in public; smoking cannabis after being found with the drug repeatedly ; possessing the drug inside or near places where there are children ; using the drug in areas where it is causing a "local policing problem", meaning a "fear of public disorder".