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Generally speaking Hibiscus like sun, warmth, rich acidic to average pH soil and plenty of moisture. Perhaps try a "Bloom Booster" type of fertilizer.

With an apparently healthy, vigorous plant that you show, one or a few flowers picked off will not affect the development of other flower buds. Picking off a large amount of undeveloped flower buds or extreme reduction pruning will limit the flower development.

It is still alive, for sure. It may be damaged by the harsh winter, and can take up to a full year to recover. Don't give up on it just yet. I would also make sure to mulch during the winter, as this will help give some protection against the cold.

Dig a hole at least twice the size of the root mass on each of the dormant bare-root plants. Partially backfill the hole and position the root ball in the hole. Position the plant so the previous soil line will be even with ground level. (The stem/trunk will be darker in color below the original planting line and lighter in color above it). Once the plant is positioned at the proper planting depth, begin filling the hole with soil. Work the soil around the roots with your hands. When the hole is half filled, tamp the soil to remove any air pockets. Fill the planting hole with water and allow it to soak in. Straighten the plant in the hole and finish filling with soil. Leave a saucer-like depression around the stems to allow for moisture accumulation and retention. Water thoroughly after planting.

After 3 weeks the bare roots should be waking up, I would suggest contacting the seller.

https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/blue-hibiscus-information.htm

Since Hibiscus blossoms only last 24 hours, the black color you describe may be normal.
This article may help you.
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-flowers-hibiscus-blossoms-falling-off-plant.htm

A hibiscus with black or brown spots on the undersides of leaves is likely suffering from an infestation of black scale insects or a leaf spot disease.
Because they don't move around, scale may not initially be recognizable as an insect and may be mistaken for a disease. Black scale insects secrete a sticky honeydew which attracts a black sooty mold.
Prune heavily infested branches. Prune also to allow more air and light into the canopy as this can help kill young scale insects, who are sensitive to heat and light.
Bacterial and fungal leaf spot can cause black or brown spots on both the upper and lower sides of leaves. Though unattractive, these spots rarely cause serious harm to the plant and often go away when the weather warms up. To expedite the process, remove heavily infected leaves and branches. Sweep up any fallen leaves. Leaf spot diseases are encouraged by wet conditions, so avoid overhead irrigation and water in the morning so that splashed leaves have time to dry out before nightfall.