I have heard that Dandelion (Taraxacum) is good for some diseases, but is it safe to use it after it blooms?

I'm gonna guess it is still safe.  The leaves will be bitter after the bloom, but the bright yellow heads are often picked to make dandilion wine, so it can't be toxic.  Don't know if I would care to try the milky stem tho.

A dandelion is a short plant, usually with a yellow flower head and notched leaves.

A dandelion flower head consists of many tiny flowers. The dandelion is native to Europe and Asia, and has spread to many other places.

The dandelion is also known by its generic name Taraxacum.

 Uses of Dandelion--
 
While the dandelion is considered a weed by many gardeners, the plant does have several culinary and medicinal uses.

Dandelions are grown commercially at a small scale as a leaf vegetable.

The plant can be eaten cooked or raw in various forms, such as in soup or salad.

They are probably closest in character to mustard greens. Usually the young leaves and unopened buds are eaten raw in salads, while older leaves are cooked.

Raw leaves have a slightly bitter taste.

 Dandelion salad is often accompanied with hard boiled eggs. The leaves are high in vitamin A, vitamin C and iron, carrying more iron and calcium than spinach.

Dandelion flowers can be used to make dandelion wine.

The recipe usually contains citrus fruit.

 Another recipe using the plant is dandelion flower jam. Ground roasted dandelion root can be used as a coffee substitute.

Drunk before meals, it is believed to stimulate digestive functions. Sold in most health food stores, often in a mixture, it is considered an excellent cleansing tonic for the liver.

 
 Medicinal properties of dandelion------

Dandelion root is a registered drug in Canada, sold as a diuretic.

A leaf decoction can be drunk to "purify the blood", for the treatment of anemia, jaundice, and also for nervousness.

The milky latex has been used as a mosquito repellent; the milk is also applied to warts, helping get rid of them without damaging the surrounding skin.

 A dye can also be obtained from the roots of the plant.

A new mixture of roasted roots is sold as a product called DandyBlend which tastes like coffee after the inulin in the dandelion is roasted.

"Dandelion and Burdock" is a soft drink that has long been popular in the United Kingdom with authentic recipes sold by health food shops. It is unclear whether cheaper supermarket versions actually contain either plant.

It seems to be a safe plant.