Can I upgrade to 5.3 ?

My ATV2 (jailbroken) is still on 5.0.2. I don’t have the SHSH blobs for 5.3.

Do you know if I can still upgrade it to 5.3 ? Is the firmware still signed by Apple or does Seas0nPass override this ?

Yes, you can still upgrade. Apple is still signing 5.3 so SP will automatically upgrade and jailbreak to 5.3 without saved blobs. You should save your 5.02 blobs using iFaith (windows only) for future use just in case.