As of September 1st, to acquire a firearm barrel, a Canadian must have his Possession and Acquisition License (PAL) verified by the Canadian Firearms Program (CFP). This is a Federally issued license permitting the possession and acquisition of certain firearms.
Suppose I am going to give a firearm barrel to a friend. I call the CFP and give them my friend's PAL information, plus some other data. The CFP confirms that the PAL is valid, and I can give him the barrel.
I am going to assume that to clear a barrel into Canada, a similar process would have to be done before the barrel would be released. This is nothing to do with export from the US, but rather with import into Canada.
To the best of my knowledge, the law refers to firearm barrels. I have no idea if that term includes barrel blanks.
In Canada, some pre-1898 firearms and some post 1897 reproductions are legally antique. They can be transferred without the recipient having a PAL. Antique firearms are not non-guns; they are firearms with antique status, and are exempt from many of the restrictions placed on modern arms.
Note that no handgun that does not qualify as an antique may be transferred in Canada. Possession is frozen. No handgun made after 1897 is classified as antique; some made before 1898 may be, but not all. Current policy is that a licensed Canadian may not make a firearm without a manufacturing license, with the exception of a match, flint (touch hole fired) long gun. A post 1897 percussion long gun does not have antique status, therefore a manufacturing license would be needed to make one.
I assume that something like a Kibler kit would be considered to be an antique firearm, and as a flintlock would not be subject to the PAL requirement and so would not be a problem to import.
The good folks at the Canadian border are under a lot of pressure to curtail gun smuggling. Anything related to firearms may receive close attention. I would be cautious about vague, generalized declarations. Might sail right through or might attract extra attention.
The legal situation is a true can of very complicated worms.