Regarding the legality of demanding a blank check as security
Regarding your question, "Whether a blank and signed check without a date can be demanded as security from an employee before joining," there is no restriction on making verbal demands. However, the employee must ensure not to accept any verbal demands from the employer. All aspects of the employee-employer relationship should be documented, whether through an offer letter, appointment letter, or a written agreement.
When a blank check is requested from an employee as per the offer/appointment letter or agreement, it becomes void in terms of the law. The employer can request security in the form of cash, a check/draft (properly completed in all aspects), or a bank guarantee. If the validity date of the check or draft is approaching expiration, the employer can request the employee to replace the old/stale check with a new check/bank draft. A bank guarantee is much safer in the interest of both the employer and the employee, with lower chances of illegal use compared to other methods.
Please let me know if you need further clarification or assistance.