I'd also like to add the fact that the tips of the leaves are turning black, and seems to be spreading towards the steam. Thanks :(

I have no idea what Salvia you have--there are hundreds, native and (mostly) non-native. But I would guess "yes" and "yes." The Salvias I know, like Salvia azurea grandiflora, like well-drained, dryish soil. And if you are using "Miracle Gro" potting soil, doesn't it already have fertilizer in it? There would be no need to add any more! I'm no plant expert by any means, but black spots to me would indicate some kind of mold, which LOVES moist humid conditions. So try giving the poor things some sun and air. Good luck!

I can sum up all I know about native salvias with these two examples: Salvia lyrata or Lyre-leaved sage has beautiful blue, flowers in May. It is great for naturalizing, will definitely spread, after blooming you can mow over it and it won't hurt. If you allow it to go to seed, finches love the seed heads.
Salvia coccinea has great red flowers Summer and Fall. Put in a sunny, open area, they are carefree and reseed themselves. Great for hummingbirds.
Its mid-June, why haven't you planted your salvias out? Most salvias are native to dry, prairies and open places and are not plants requiring rich soil and fertilizers.
Sam

i agree that you should stop fertilizing, remove the bag, and put the plants outside. I would not put them directly into full sun, since they are accustommed to lesser amounts of sunlight, but I would move them out. You probably have a non-native salvia, most likely a cultivar of Salvia elegans. This and most other salvias normally sold in nurseries will do well with moist, rich soil, but the bag and constant excessive fertilizer may be encouraging mold. Also, the temperature inside the bag may get really high when the sun is shining on the plants, which may be part of the problem. Most salvias do well in the ground over summer, but they can also do well in large pots.

i would not overcompensate by trying to switch to dry conditions immediately. Keep the soil evenly moist, but make sure it is well drained- make sure there is no standing water under the pots. Once the plants are outside, they may require water every day in the summer, as do most potted plants.

Ladyslppr is right in her recommendations. I wish to add the following:
The dark coloration you are experiencing is due to loss of root because the plant roots are being kept just too damp. The miracle gro soil mix is NOT the best for Salvias. Salvias require fast draining highly aerable soil mixes and are probably best in soil mixes that are not enriched by fertilizer additives.
For a mix use either mix for Cactus plants-though avoid any such mix if it has peat moss or sphagnum in it. Your best bet is to mix your own. Try the following:
2 parts river sand
1 part granitic gravel
2 parts acid loving plants soil mix
and 1 part either hard wood leaves such as pin oak or fine grade bark (such as ground cover bark)
What is important is that the plant should be underpotted.
Stuff the roots into a pot that is only a little larger than the existing root ball-but shake off any of the old soil you can but do not disturb the roots physically.
Pour in the new mix dry. water. add more dry as it will settle and water again and repeat until the soil level is at the base of the plant's stem(s)
Put it outside with 50% light and as long an exposure as possible (4 to 6 hours).
Salvisas enjoy the sun and the heat. Do NOT water regularly. but only when the pot feels much lighter even though the plant does not appear to be slightly wilted. Sometimes I wait until the growing stem start to droop a bit before watering.
Currently I grow Salvia apiana in a pot. Can supply an image if you wish to see it.

Here is a link that might be useful: Caifornia salvias