Why is porphyrin aromatic?
The aromatic properties are evident from the heats of combustion, the fact that all bonds have the same length, the ability of porphyrins and metalloporphyrins to undergo electrophilic substitution reactions, the stability of the porphyrin nucleus in electron impact mass spectrometry and the characteristic chemical shifts observed in proton NMR spectra1. 

The 18π electron substructure (shown in blue in your figure) was thought to be responsible for the aromatic characteristics. In support of this view it was noted that the outermost bonds in the pyrrolenine rings(the not blue double bonds in your figure) take part in cycloaddition reactions(Fig.1, taken from ref.2) and therefore have a significant double bond character suggesting that their electrons are more localized. 

This 18π model is also called "the diaza[18]annulene model" because it was speculated that the aromatic pathway found in porphyrins resembles [18]annulene(fig.2, ref.1). You can also note the similarities in the spectra of [18]annulene(fig.3, ref.3) and porphyrins: both have a major peak in the UV region and a less intense absorption at longer wavelengths.  

Other supportive data to the the diaza[18]annulene model are proton NMR studies all mentioned in ref.1


https://www.worldscientific.com/doi/abs/10.1142/S1088424611004063
http://www.scielo.br/pdf/jbchs/v15n6/22664.pdf
Chem. Commun., 2016, 52, 4710