The blue 'V' means you have a QSO verified after presenting the QSL to ARRL for an award.
When you upload an ADIF, it will generally include details of any QSLs you have entered into your logging software. Club Log can use this information to show you reports that help you focus your QSLing efforts.
There are three possible flags in the ADIF file that Club Log is able to interpret:
QSL_RCVD = N
This means you haven't had a card, so the QSO is unconfirmed. This is referred to as 'worked' and is signified by a yellow 'W'.
QSL_RCVD = Y
This means you have a card, so it's confirmed. This is signified by a green 'C'.
QSL_RCVD = V
This means you've presented the card to ARRL and received credit for the QSO. This is signified by a blue 'V'. In ADIF 3 files, the 'CREDIT_GRANTED' field is used (Club Log will only apply credits for DXCC claims made in this field).
Important: 'V' is only used when an award has been paid for in LoTW. A match in LoTW is just 'C', not 'V' until you choose to apply for the credit as part of an award application.
Not all logging software supports this field, and it may also require additional effort on your part to make sure you enter the verifications in your log.
LoTW is also considered at the same time, using the LOTW_QSL_RCVD field.