Not every blockchain is invisible. There's been 51% attack on Ethereum Classic (ETC), Feathercoin (FTC), Bitcoin Gold (BTG), Vertcoin (VTC) and Verge (XVG).
Although, there's ways of disrupting the network such as the implied 51% attack, it doesn't really effect the Blockchain, especially not long term. As suggested, the Blockchain is merely a public ledger, which stores information that's broadcasted to it. That's all. The 51% isn't a hack either, its more of an attack to disrupt the network, although its not a very feasible attack for the long term. Although, your point is somewhat true. Altcoins, and generally lesser known blockchains are more likely to suffer form 51% attacks, due to the fact that there's less devices contributing to the hashing rate.
