Let me start by saying that I'm no expert. I've just done a lot of reading, and I have an opinion. So here I go back on the soap box... I apologize about the length up front.
As far as foliar sprays go, the theory is that leaves respire/breath. Because they have pores, they can absorb nutrients/fertilizer like roots can. Foliar sprays are supposed to get needed nutrients to the upper part of the
plants faster than going from the root up the stem to the rest of the
plant. That's the theory.
Does it work? I can't endorse or object to it. I've bought several foliar sprays. I just haven't gotten around to using them as much as I planned on, and I haven't done any experiments to see if they do make a difference. At a minimum, even if the theory isn't quite right, you're putting fertilizer on the
plant that will probably drip off during application or wash off during rain. That "run-off" will eventually make it's way into the soil and into the
plant.
Most orchids have very waxy leaves probably to keep them dry in a rain forest or tropical climate. I'm not sure there would be much leaf absorption on
plants where the water beads up on the leaf surface or runs off. I did come across a product called Coco Wet that is supposed to reduce the surface tension of the water so it doesn't bead up that allows any foliar spray to cling to the leaf surface better. Again, bought it, but haven't used it, so I can't endorse it.
Can you be more specific about the black fertilizer pellets? Does it have a name? Does it have the N-P-K rating? I won't even go into what is considered organic, that's a whole other thread.
There are lots of organic fertilizers out there. You can buy bat guano (poop), coir (shredded coconut fibres), fish emulsion or volcanic rock powder to name some of the more exotic ones. Lime to reduce acidity, sulfur to raise acidity, worm castings (poop), peat most (non-renewable resource), and composted manure are more common and readily available.
The thing with any fertilizer, chemical or organic, is that it's not one size fits all (except maybe compost). The numbers for Nitrogen, Phosphorus and Potassium are there to let you know the concentration. When it comes to fertilizer, you can over do it. If you add a high nitrogen fertilizer to soil that is already high in nitrogen, you can burn the
plant or cause explosive growth followed by weak
plant health. It's recommended that you add fertilizer to correct deficiencies, or to meet the needs of specific
plants not just to fertilize for fertilization sake.
For example, corn is a heavy nitrogen feeder. If you planted corn last year, the nitrogen in the soil may be depleted. So using a high nitrogen fertilizer to replenish the soil is appropriate, or you can grow a legume crop to add nitrogen to the soil instead.
Another example is if you have a cold and you're feverish and achy, there is no good reason to take a multi-symptom cold medication with a decongestant and cough suppressant. You would/should take the medication appropriate for your symptoms.
With that said, potted
plants do need fertilizer. Their ecosystem is limited, and not renewable. We attempt to create an artificial environment that replicates the native one as much as possible. Like the orchids you mentioned, they won't thrive outside of their natural habitat if we don't fertilize.
I guess my point is if you are going fertilize, do it when appropriate only if you need to with the nutrients you need. If you find a foliar spray or soluble fertilizer (organic or chemical) that works for you, just make sure that you are not over fertilizing.
I guess that's why I like compost. I've bought soil test kits, and while it's fun playing chemist, it's far too complicated and time consuming for me. Compost builds soil structure, improves water retention, and adds nutrients back into the soil. It buffers against the effects of too high or too low pH. It comes from the natural decomposition process, and organic materials. It is full of life and micro-organisms. Depending on the materials used, the N-P-K can vary, but because it is "alive" it can make all nutrients available in the soil including micro nutrients.
That's just my two cents, and again I apologize for the length.
