how to take down blackhat site ddos script

It depends... you could report them to their ISP but since ip addresses can be spoofed it is rather difficult.. Cloudflare.com might be of assistance in withstanding the DDOS attack.. they have the bandwidth to throw at the DDOS where you probably don't.

Good guide as in the reporting blog from ICANN share the below
I’m under attack. What should I do? Whom should I call?
Helping Hands
Should I contact Law Enforcement?

A quick ddos quick checklist (pdf) for mitigation, the reporting is just reactive and probably for long term but to be more proactive the ISP, your infrastructure network devices like FW/App FW and App cum servers need to measure up to the ddos in resources exhaustion and exploit.

(note you cant possible counter ddos 100% as those likely are botnet or victim machines - just look at recent Microsoft helping FBI to contain and sinkhole Gameover/Zeus/Cryptokiller, but there can be repercussion if such attempt is not execute properly and with care as in MS recent no-ip efforts)

couple of ddos service include cloudflare, akamai, incapsula, arbor cloud, etc. Even opendns can come into picture as well as content delivery n/w (CDN) to impede and slow down the ddos launches which can be from botnet of varying IP addresses..dns sinkholing and to extreme geo blacklisting can be employed if deemed critical

Even if you do correctly identify a DoS or DDoS attack, it is unlikely that you will be able to determine the actual target or source of the attack.

And contact your own isp to see if they can help you, either in mitigation or in upstream reporting

Unless you are determined (have time) ditto noci, your own ISP may not want the traffic and have concern to report back up the chain. Likely the site is surrogate, or will move on.