hi everybody

one of my client bought five different domain name which are very relevant to his business and and highly competitive keywords.

i'm thinking to host these domains on my host with unique but relevant content.

i want to know that is it called blackhat seo?

It's not as such blackhat, but you could probably acheive the same positive ranking effects for the target site with much less work.. the sites you will be linking from will have no backlinks of their own, so although the relevency might be high there will be little juice to pass up.