Ashley, who owns Sports World International and the landmark Lillywhites store at London's Picadilly Circus, now has a 29.4 per cent shareholding in Blacks - just shy of the level required to trigger a formal bid.
Broker Numis Securities speculated that a bid could be worth as much as 500p a share, valuing the company at£212 million.
However, Seymour Pierce said that it was unlikely any offer would be above 450p a share.
Both emphasised that Ashley has a habit of picking up stakes in rival businesses, such as JD Sports and JJB, but not taking things further and making a takeover approach.
The confirmation of his stakebuilding in Blacks came after the retailer yesterday revealed that pre-tax profit had crumbled to just£100,000 from£6.9 million, with like-for-like sales down 0.5 per cent.
The retailer blamed 'weakness' at its Millets chain and investment in store space and infrastructure.
In morning trading Blacks Leisure was trading at 448p a share.