I've found blocking to be really useful for my small-scale experiments for 2 different reasons:

1. Often, I'm worried about simple randomization leading to an imbalance in sample vs experimental; if I'm only getting 20 total datapoints on something, then randomization could easily lead to something like 14 control and 6 experimental datapoints - throwing out a lot of statistical power compared to 10 control and 10 experimental. If I pair days, then I know I will get 10/10, without worrying about breaking blinding.
2. Blocking is the natural way to handle multiple-day effects or trends: if I think lithium operates slowly, I will pair entire weeks or months, rather than days and hoping enough experimental and control days form runs which will reveal any trend rather than wash it out in averaging.