The +1/+1 counter isn't put on Markov Blademaster in time to increase the amount of combat damage dealt during that combat step. However, the +1/+1 counter put on Markov Blademaster when first-strike damage is dealt will increase the amount of damage dealt during the regular combat damage step.

If this creature deals combat damage to multiple players simultaneously, perhaps because some combat damage was redirected, its ability will trigger for each of those players.