Well, first I need to note that blkdiscard is not more secure. But it is much more faster. It does not actually wipe flash memory, it just tells the controller to mark it as unused. So it will drop stored data at the moment it decides the best. Maybe immediately, maybe just before writing new data. But anyway it wont provide ability to read it. It would be still possible if you can get direct access to the flash memory bypassing the controller.

Second, you forgot that SSDs are not HDDs and data are not stored exactly at offset you write them. The controller remaps memory blocks as needed. And it has more blocks than actually available to user. So when you use dd (or cp, or any other program writing directly to block device) you only override blocks that are actually mapped, but some blocks can still keep old data. So using dd is also not secure in case someone can get direct access to the flash memory. But it takes much longer time and reduces the flash lifetime.

Several people here mentioned a secure erase feature of SSDs. I didn’t know about it. It should be more secure than both methods if implemented correctly by the manufacturer (i. e. clears all memory cells immediately). In the worst case it could be the same as blkdiscard, I guess.