Why hydrogen lines are less visible in the Sun spectrum than in supernovae clouds? Supernovae clouds are very colorful, and if I trust documentaries I watched, the colors are due to excitation of elements, as in fireworks. Since the Sun is mostly made of hydrogen, I suppose those lines should be very apparent but they are not so much, its light looking like a blackbody radiation. What contributes to the rest of the spectrum up to the point it masks hydrogen lines?
 A: The sun's spectrum is a blackbody spectrum with absorption lines superimposed. The blackbody spectrum comes from the photosphere, which is the highest elevation at which the sun is opaque. It becomes opaque because it's ionized to form a plasma. A plasma is opaque because it has free charges that interact strongly with electromagnetic waves and absorb their energy. The spectrum is a continuous blackbody spectrum rather than an emission spectrum because it's a dense plasma. Discrete emission and absorption spectra are typical of thin gases composed mainly of neutral atoms or molecules.
The absorption lines in the sun are created by layers farther out in which the gas is not ionized and less dense.
A: 
What contributes to the rest of the spectrum up to the point it masks
  hydrogen lines?

It's basically due to the optics.
In the case of the sun, you're seeing the energy of the gas in the outer shell. This is highly random, moving in all directions and over a huge distribution of speeds. So everything is being blurred out in frequency by Doppler shifting, which is why...

its light looking like a blackbody radiation

In the case of a supernova remnant, you're looking at a bubble of hot gas caused by the outward moving particles shock-heating the interstellar medium when it hits it. So, in this case, in any one area of the cloud, you're seeing particles that are moving at a much more limited range of speeds. This is also why you see a sort of color shift, notice that the crab nebula looks redder at the edges than the center.
A: The difference is that in the Nebulae you have black background for your hydrogen lines emission, and you can observe this easily with very simple imaging techniques (typically, 3.5-8nm pass-band filters do just fine). 
On the sun you have "white" background - black body emission from deep levels of the sun. Hydrogen lines are now visible as absorption lines - darker than background. But in order to see them with enough contrast on this white background you need much more sophisticated equipment - ~0.1nm pass-band filters, otherwise there is too little contrast to see anything. 
So the answer is simple: Hydrogen emission lines on black background have much larger contrast than absorption lines on white background. Hence emission lines in nebulae are much easier to observe and photograph.
