Black Orchid restores your horse's health, energy, and morale. However, if the horse is over 25, over/underweight, they will continue to lose health as they age. The Orchid's effect is temporary.
With the Heel, your horse will never lose health from having too little energy or getting put to bed past 22:00. This does not prevent the horse from beginning to lose health at age 25 if they are mortal.

The Heels work day after day. They take away health, morale, and energy penalties for putting the horse to bed after 2200 or with less than 20% energy.

The Orchids give a one time full recovery of health, energy, and morale. You can put multiple orchids on the same horse but the effects of each are a one time thing.

I personally don't use orchids much unless I happen to buy one that's severely underweight and I'm afraid they'll die when they age. I love the heels though. I put them on almost everything that's in training or that I enter in lots of comps.