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Axion-like particles (ALPs), which are very light neutral spin zero elusiveparticles primarily interacting with two photons and predicted by superstringand superbrane theories, have come to help by solving two distinct problemsabout blazars (a type of active galactic nuclei), thus providing two hints atthe existence of ALPs themselves. In the presence of an external magneticfield, ALPs produce: (i) photon-ALP oscillations, (ii) the change of thepolarization state of photons. The former effect has many consequences in theastrophysical contest, such as the modification of the transparency of theUniverse and the alteration of the astrophysical spectra. We address here thelatter effect by analyzing how the photon degree of linear polarization and thepolarization angle get modified by photon-ALP interaction in the case wherephotons are generated at the jet base of some BL Lacs (a blazar class): OJ 287,BL Lacertae, Markarian 501 and 1ES 0229+200, by considering both a leptonic andhadronic emission mechanism. We show that OJ 287 and BL Lacertae are goodobservational targets for ALP studies both in the X-ray band with IXPE (alreadyoperative) and with the proposed eXTP, XL-Calibur, NGXP and XPP missions and inthe high-energy range with the COSI, e-ASTROGAM and AMEGO missions, while 1ES0229+200 represents a strong candidate in the X-ray band only. Since theseblazars show a very high final photon degree of linear polarization, whichcannot be explained by conventional physics, such a possible detection wouldrepresent an additional hint at the ALP existence. Instead, Markarian 501 doesnot appear as a good target for these studies. We conclude that all theseobservatories can give us additional fundamental information about ALP physics.

ALP induced polarization effects on photons from blazars