How can i extract unmapped reads using blast

Hello everyone

i have questions please

my data is contaminated by host mitochondrial DNA , and i need to do mapping of my data against host mitochondrial DNA and extract unmapped reads to do assembly next

first : how can i download just mitochondrial DNA and not the entire genome of the host speceis ?
second , can anyone please tell me the blast command i should use ?
third : how can i obtain unmapped reads ?

really do need your help and Thank you so much .