Bitcoins are secure means of investment nobody can hack into bitcoins if you store them in an offline wallet it is the best way to store them in a paper based offline wallet trezor and ledger nano S are some of the safest wallets in which you can keep your bitcoins safe as far block chain is concerned nobody can hack into blockchain as the programmer has made it so secure so that nobody can easily hack into blockchain as btc runs on the support of blockchains .Bitcoin is not blocked for those who use them illegally as there is no idea on who the user is as they maintain complete privacy of the user name to identify the user.