Where to connect BLK and GRN pins of Openlog board on my Uno?
If you read the page about the OpenLog board you can see (on the schematic link) what the pins mean:



I've added the labelling on the board in blue.  The GRN pin (DTR in the schematic) is actually an input, which resets the chip:



Thus:


GRN should be connected to RESET on your Uno, so that when you reset the Uno, you rest the OpenLog.
TXO (transmit out) on the OpenLog would be connected to serial input on the Uno (ie. digital pin D0 - labelled RX).
RXI (receive input) on the OpenLog would be connected to serial output on the Uno (ie. digital pin D1 - labelled TX).
You can see that GND and BLK are both ground pins, and thus should be connected to ground.
VCC (RAW on their schematic) is the voltage input to the 3.3V very-low-dropout voltage regulator MIC5205 on the OpenLog. This will accept input voltages in the range 3.3V to 12V. (The datasheet seems to suggest a maximum of 16V, but it might get rather hot if you did that). Connecting VCC to +5V on the Uno would be satisfactory.