In a small bowl, whisk together olive oil, mustard, pepper, salt, poppyseeds and vinegar until emulsified. Taste test and add more salt if needed.
In a large mixing bowl, toss together spinach, blackberries, cheese, pecans and dressing. Serve immediately. If prepping ahead of time, wait until just before serving to toss with dressing.
Notes
Dressing will keep in the fridge in an airtight bottle for 5-7 days.