"if" someone does have high bp, do the readings usually stay high the whole day/evening or do they lower at times to "normal".

Fluctuate. BP normally fluctuates. High BP is defined as "basal" bp, that is, BP that remain elevated, on average, at complete rest in a calm comfortable setting. BP is dynamic (always changing) and high levels during exercise is healthy/physiologic. High levels at rest are not. Conversely, BP averages are used, so occ. Normal pressure at rest with a high average still needs to be controlled.