Of course it's still valid bug for 4.x since the particular code hasn't been touched in any helpful manner.
The cause is related to the following issue: https://jira.spring.io/browse/SPR-11125 which in short is that the context classloader may become different than what you optimistically expect (because of what's described in that linked issue) and because your code just checks whether it's null, it won't get it right. The issue affects SerializableBlobProxy and SerializableClobProxy getProxyClassLoader methods. The quick fix is to skip obtaining the context classloader and just use the 2nd option - classloader that loaded the WrappedBlob.class etc.