seedlings ended up long and spindly

That particular issue is caused by insufficient light and overly warm ambient air temps, primarily lack of light, and not by the growing medium used.

Seedlings require intense, broad-spectrum light from the instant of germination for a minimum of 12-16 hours out of 24. Low intensity and/or lights too far above the plants causes them to stretch toward it and ultimately fall or die as the stem cells are stretched. Other than over-watering, it is probably the most common problem associated with starting seeds indoors.

Overly warm environmental temps (air, not soil temps warmer than 60-65 degrees) only make the problem worse.

There is a FAQ here and many discussions about the various types and amounts of lights to use including plain old shop lights kept no more than 1" above the plants but the bottom line is that you literally cannot have too much light and if plants get leggy then they aren't getting nearly enough.

keep them in the dark until they sprout

Only a few flower seeds require darkness to germinate. 90% of seeds - flower or vegetable - do not.

As for medium to use, Rapid Rooters or any similar product works fine for an ebb n' flow. I personally prefer rock wool cubes but only because they retain their form better and aren't as fragile.

Dave

SEED STARTING FOR THE HOME GARDEN
I have discovered a cheap, simple method of starting flower and vegetable seeds for the home garden. It utilizes the principle of capillary action and a moist newspaper mat. To construct such a device, the following items will be required:

1. A roller paint tray and plastic liner obtained from the paint department of a box store.
   2. A 5-foot length of 1ü inch PVC pipe obtained from the plumbing department of the same store. This needs to be cut into 25 2-inch cylinders and then filled with a good seed starting mix.
   3. A 6-inch piece of I-inch PVC pipe to be used as a plant ejector.
   4. A half dozen sheets of newspaper.
   Lay the newspaper on the sloping part of the paint tray with several inches immersed in the paint holding well. Fill this well with water and watch as the newspaper mat slowly becomes saturated as it draws up water. Plant a few seeds in each mix-filled cylinder and tamp lightly so the bottom of the mix is in contact with the wet newspaper. It will soon start drawing up moisture via capillary action and become damp. Keep adding water to the well as needed.
   Using short pieces of an old venation blind and a lead pencil, identify each cylinder with name and date of planting.
   In a week or two, the seeds will germinate and green leaves will appear. When about 2 or 3 inches tall and ready to transplant to a larger container or the garden, use the 1â ejection tool. Place this over the green leaves of the sprouted seedling. It will telescope nicely into the 1üâ planted cylinder. Pull up on the planted cylinder and the seedling will be bottom ejected without disturbing the roots. Add fresh planting mix around the new, larger container and place in a bright-lighted window for further development.