Hibernate 3.5 has changed the way it deals with LOB and CLOB fields as part of it's move to support for JDBC 4. This is briefly touched on at http://community.jboss.org/wiki/Hibernate-JDBC4support
There's not much point in overwriting the jar.
To poke around a bit in some of the code involved, see the org.hibernate.engine.jdbc package in hibernate-core.jar, perhaps start with the LobCreator interface and see usages and implementations of it. That's what I did before responding here. If you have questions about how to migrate your app to remove what caused direct use of the SerializableBlob class, I suggest you try the user forums at hibernate.org or perhaps the #hibernate room at freenode. You're more likely to get an answer there, although perhaps someone will answer here.