I know that this question is almost 3 years old, but spraying against powdery mildew with a 10% milk solution?! Jesus Christ, please, no, don't do that! Elemental sulfur (during shady hours) and milder copper solutions (e.g. copper hydrochloride) are the ONLY remedies to this problem, not old wives' tales!
That being said BER is a problem caused by multiple factors. The problem is not with a "critical period" or a specific stage of the produce, but the environmental factors. Some cultivars are more susceptible to this problem than others (so you might want to pick your seeds/seedlings accordingly this year), hot days with no (or little) water might increase its likelihood and so does RELATIVE calcium deficiency (usually not the lack of calcium in the soil, but the plants' inability to absorb it). Other than that the problem might manifest at any stage of the produce all the way until it's fully ripe (in my case one of my jalapeno chilies has started to rot while being unripe while the others that came after this one has been cut off were unaffected). I'd say that if you know that a certain plant in your environment is susceptible to BER (because of your soil, your climate, the length of your dry spells etc.) you might want to give a foliar spray of calcium nitrate to your plants every 2-3 weeks to prevent this problem from happening (once the symptoms appear, there's nothing you can do).