I had these, recurring pretty constantly til DS was 9mo. They were little grains of hard milk, almost chalky.
Best thing I found was to get in the shower, massaging the blocked side down towards the nipple under the warm water so the milk pressure pushed it out, this often worked, or immediately after the shower latching DS on so he could clear it.
I would often have to 'scratch' the top of the nipple gently with my fingernail to loosen the skin over the duct a bit, then hand express until the blockage passed. And oh the relief when it came out!!! :)
Sometimes a hot flannel over the sore boob before feeding helped the baby clear it too.
I also had some far more tricky ones, which I had to use a needle to clear. This sounds extreme, but i'd read it online and checked with my doc before trying. Basically I sterilised a needle/pin and used it to clear the duct opening and 'flick' the grain out. Not something I'd advise you try until all other avenues have been exhausted! Like I said tho, I had this a LOT for many months, which is unusual.
I found wearing a bra at night would often cause me to wake up with a blocked duct, so maybe they're caused by pressure.
Hope you get some relief soon.