Blue light blinking after replacing battery!

Some troubleshooting thoughts:

• Let the phone charge for at least 30 minutes to see if this fixes the issue.
• Follow the iFixit guide to make sure the battery connector is fully secured: Samsung Galaxy S6 Akku ersetzen
• Clean the charging port with compressed air to see if any debris is keeping the phone from charging.
• Try a different charging cable.
• Try a different wall outlet (or USB port).

You could try a factory reset to see if this fixes the issue. However, a factory reset would completely wipe everything you have on the phone, which is typically not desired.

Further troubleshooting may be dependent on the original reason for the battery replacement (water damage, not holding a charge, swollen).

Since both batteries are not starting up the phone, it may be that a) there is a deeper problem within the phone itself or b) both batteries do not hold a charge.