I was recently using a Mifare Classic 1k smartcard. I used the manufacturer block (sector 0, block 0) to identify the card, since the UID of Mifare Classic cards is not really unique.
My application scenario is that I want to read an identifier from the smartcard and compare it to a database, in which every user has a smartcard assigned to them. So a user can log in with their card.
Now I have to use new Mifare DESFire EV1 4k cards. Do these cards also have something like the manufacturer block or just the 7-byte UID? Is it safe to use this UID to identify the card?
You can expect the 7-byte UID of genuine MIFARE DESFire EV1 cards to be unique. Be aware, though, that DESFire EV1 cards can be configured to use a random 4-byte UID. In that case, the UID is randomly generated for each RF activation. The 7-byte UID of such cards can only be obtained using the GetCardUID command (command code 0x51) after authentication.
However, you should be aware (in particular considering your use case) that the UID can easily be cloned and is not suitable to authenticate cards (or their users). See also
If you want something that is compareable to the manufacturer block of MIFARE Classic (i.e. something that reveals information about the product type and the production batch), MIFARE DESFire cards have a GetVersion command (command code 0x60).
Finally, note that the manufacturer block of a MIFARE Classic card does not really provide more uniqueness than the UID on its own. The reason for this is that the first 4 bytes of the manufacturer block are the UID itself. The next byte is the checksum over the UID (so its value is the same for cards with the same UID). The next 3 bytes are the SAK and ATQA values, which are typically '88' and '0400' for all (most?) MIFARE Classic 1K cards. The remaining bytes identify the chip type (hence identical for all cards of same type) and the production batch (hence identical for at least each batch).