if an amendment violates the constitution,why should the Supreme court block its ratification?

How can an amendment violate the constitution? THe amendment once adopted is a PART of the consititution.
Now proposed amendments are all unconstitutional things, if they had been (and usually were) laws. Why else would they be proposed changes to the Constitution?
There is one exception to the SC getting into the fray and delaring the passed amendment unconstitutional: that is when the amendment was passed in an unconstutional manner.