How do I update a point feature class that was transformed from a table with X/Y coordinates in an ESRI SDE geodatabase?

I'm not quite sure what you are trying to do. Are you trying to update existing records ​in your sde so they have a point geom using the bl_id? Looks like its saying there is no corresponding record with that bl_id value in the table and therefore can not update

I would use a feature reader to read your point features from sde table in alongside your original tanle and then use feature merger or joiner or change detector to check back to original table if the feature exists which would be an update to the sde table , if it doesn't exist then it's an insert and if it no longer exists in original table it needs to be deleted from the sde , the different ports will indicate what the feature operation should be. The feature operation would be set accordingly for each type of operation you require. Check out this article it should guide you on how to deal with multiple operations in the same workflow https://community.safe.com/s/article/updating-databases .

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A​t the moment your writer is set to update for all features which is why you aren't getting new inserts . You just need to build a method to distinguish between the different operations and create an attribute called fme_db_operation and set the value as UPDATE INSERT or DELETE and use that attribute in the feature operation parameter in the writer to change the type accordingly. Hope that helps