Inexperienced pilot of an unidentified single pilot aircraft encountered 'extreme' turbulence when penetrating a dark cloud in an area of convective weather.

Narrative

My total actual IMC logged time is only about 50 hours; so I'm relatively inexperienced regarding the recognition of which cloud build-ups are safe to penetrate; and which ones require a diversion. During this flight; the build-ups were generally in the formative stages (early afternoon) and I easily penetrated and exited several build-ups with minimal heading or altitude change. As I approached one cloud; I noticed it appeared much darker than the previous ones and subsequently; upon penetration; I experienced extreme turbulence. At one point I believe my altitude was deviating by as much as + 300 feet; based on my altimeter. I told the controller that I was having trouble holding altitude and asked the controller for an altitude block. By the time he had an opportunity to respond to my request; I told him that I was safely through the cloud and that I could maintain 8;000 without difficulty. There was no indication that a separation conflict occurred; and as far as I know; the deviation of altitude went unnoticed by the controller. This experience suggests that I should invest in a hand held device of some sort so that I may receive NEXRAD images in-flight. Also; there should be some more focus placed on the methods employed by controllers to keep aircraft out of convective clouds; to the extent possible; as well as the voice communications used to negotiate safe passage.