Blast is "Similarity" Based Search: Homology might be implied from the Similarity. In biological context Homology is derived from common ancestry.
The Basic Local Alignment Search Tool (BLAST) finds regions of local similarity between sequences. The program compares nucleotide or protein sequences to sequence databases and calculates the statistical significance of matches. BLAST can be used to infer functional and evolutionary relationships between sequences as well as help identify members of gene families.
Homology is a conclusion of evolutionary relatedness. Sequence similiarity is a piece of data that would allow one to conclude homology.
BLAST might tell me that a bit of DNA is highly similar between a vector and an unfinished genome. But I'd be wrong to conclude homology from that!
statistically significant similarity implies homology
See the paper below:
An Introduction to Sequence Similarity (“Homology”) Searching
William R. Pearson