With any type of server, you definitely need to layer your security approach. Such as keeping the system fully patched, having least access controls in place, etc. If your seeing the muieblackcat hits in your logs, it's more than likely just a scanner hitting your system looking for php vulnerabilities (http://serverfault.com/questions/309309/what-is-muieblackcat). These types of scans will be very common for any external facing web server.
As far as blocking this IP, I would agree with Aditya as far as iptables is better than htaccess due to iptables blocking all traffic from that IP. You can also look into some other tools suck as fail2ban that can monitor log files and then create iptable block rules on the fly as it sees specific traffic hits such as too many failed password attempts.