What exactly do you need to be able to do with firstname/lastname/email data in your application?

You could create activities that opens the object and modifies this data however you require, for example.

For example, if you need to delete a customer's personal information from your system you could have an activity that opens their customer record and changes firstname, lastname, email to "former customer".  obj-saving this information would write it to that customer record's BLOB.

Hello,
our main tool is part of a E2E ordering chain. There's an external masking tool for all other standard DBs which changes E2E all sensitive data as per the masking defined X -> Y, so that in all applications, name X become Y.
On our side, we can easily do this for exposed columns but via application itself we obviously still see the real data.
We've got few TB of data, we need to perform masking in all relevant BLOB fields so here is the question (for which the answer seems no)
I was looking for something potentially existing (could be with a license) but it seems it needs to be created on our side. 
Thank you
Regards
Anthony

Any masking like you describes sounds specific to how your application and the associated enterprise requirements in Pega, so yes, you would need to create these activities that meet your needs yourself.