Q:

Does updating the data in blob inside cassandra DB result in tombstone? 

I have a table with a column defined as blob, the blob data is a collection of custom objects.  If I want to delete an item from the collection inside the blob, will it result in tombstones? 

A:

In short no. 
You will update the row with a new version of the blob.  This will not write a tombstone to disk, but it will eventually write the new version to disk when memtables get flushed. 
If you update really often you may have many versions of the row which need to be merged/checked for the most actual version.  Compaction merges sstables to limit the number of sstables and the stale versions
So keep an eye on your compactions consuming to much cpu or io.  But if anything is right you won't have old data reappear.