Mask off the blocks and tarmac if you have even the slightest concern regarding your ability to control the flow from a watering can or spouted bucket.
Very, very carefully pour the compound into the joint.
Keep well off it for at least a couple of days to prevent it being trafficked by shoes or tyres.
Alternatively, look at using an Overband tape.....there are narrow 25-30mm versions available somewhere....can't recall the name of the supplier.
You might want to check the condition of the haunching on the blocks, too. There's a hint of a suggestion that some of them may have 'slipped' over the years.