Hitler was a genius too. So what? At any rate Madison said it very clearly. Once in the Union always in the Union. The only legitimate means of getting out is through an amendment allowing it. The tenth amendment has NOTHING to do with secession in fact ALL the Founders adhored the concept. Washington's Farewell Address was primarily a condemnation of the idea and it described exactly the kind of people who would push the idea.
Secession wasn't spelled out, because most considered it a more or less basic right. Virginia expressly mentions the right, in their ratification documents.