Hello, I've never heard of that symptom for Flasback, but...
Flashback - Detect and remove the uprising Mac OS X Trojan...
In order to avoid detection, the installer will first look for the presence of some antivirus tools and other utilities that might be present on a power user's system, which according to F-Secure include the following:
If these tools are found, then the malware deletes itself in an attempt to prevent detection by those who have the means and capability to do so. Many malware programs use this behavior, as was seen in others such as the Tsunami malware bot.
The most current flashback removal instructions are F-Secure's Trojan-Downloader:OSX/Flashback.K.
More bad news...
The "Flashback" malware had nothing to do with a screen flashing. The "flash" in Flashback was related to this malware first being found in sites that prompted users to install a fake flash (as in Flashplayer from Adobe), which then caused an infection.
When does the flashing screen happen? Can you relate it to something you are doing?
my software update is not showing any update after i installed safari 6 on 30 of jul
Software Update only shows items it installed. Did you use Software Update to install Safari 6 or download it from the web site? In either case there will be a file in /Library/Receipts/boms/ named something like "com.apple.pkg.Safari..." with a date sometime in July.